.\" Page by b.hubert .\" and Copyright (C) 2015, Thomas Gleixner .\" and Copyright (C) 2015, Michael Kerrisk .\" .\" %%%LICENSE_START(FREELY_REDISTRIBUTABLE) .\" may be freely modified and distributed .\" %%%LICENSE_END .\" .\" Niki A. Rahimi (LTC Security Development, narahimi@us.ibm.com) .\" added ERRORS section. .\" .\" Modified 2004-06-17 mtk .\" Modified 2004-10-07 aeb, added FUTEX_REQUEUE, FUTEX_CMP_REQUEUE .\" .\" FIXME Still to integrate are some points from Torvald Riegel's mail of .\" 2015-01-23: .\" http://thread.gmane.org/gmane.linux.kernel/1703405/focus=7977 .\" .\" FIXME Do we need to add some text regarding Torvald Riegel's 2015-01-24 mail .\" http://thread.gmane.org/gmane.linux.kernel/1703405/focus=1873242 .\" .TH FUTEX 2 2020-11-01 "Linux" "Linux Programmer's Manual" .SH NAME futex \- fast user-space locking .SH SYNOPSIS .nf .PP .B #include .B #include .B #include .PP .BI "long futex(uint32_t *" uaddr ", int " futex_op ", uint32_t " val , .BI " const struct timespec *" timeout , \ " \fR /* or: \fBuint32_t \fIval2\fP */" .BI " uint32_t *" uaddr2 ", uint32_t " val3 ); .fi .PP .IR Note : There is no glibc wrapper for this system call; see NOTES. .SH DESCRIPTION The .BR futex () system call provides a method for waiting until a certain condition becomes true. It is typically used as a blocking construct in the context of shared-memory synchronization. When using futexes, the majority of the synchronization operations are performed in user space. A user-space program employs the .BR futex () system call only when it is likely that the program has to block for a longer time until the condition becomes true. Other .BR futex () operations can be used to wake any processes or threads waiting for a particular condition. .PP A futex is a 32-bit value\(emreferred to below as a .IR "futex word" \(emwhose address is supplied to the .BR futex () system call. (Futexes are 32 bits in size on all platforms, including 64-bit systems.) All futex operations are governed by this value. In order to share a futex between processes, the futex is placed in a region of shared memory, created using (for example) .BR mmap (2) or .BR shmat (2). (Thus, the futex word may have different virtual addresses in different processes, but these addresses all refer to the same location in physical memory.) In a multithreaded program, it is sufficient to place the futex word in a global variable shared by all threads. .PP When executing a futex operation that requests to block a thread, the kernel will block only if the futex word has the value that the calling thread supplied (as one of the arguments of the .BR futex () call) as the expected value of the futex word. The loading of the futex word's value, the comparison of that value with the expected value, and the actual blocking will happen atomically and will be totally ordered with respect to concurrent operations performed by other threads on the same futex word. .\" Notes from Darren Hart (Dec 2015): .\" Totally ordered with respect futex operations refers to semantics .\" of the ACQUIRE/RELEASE operations and how they impact ordering of .\" memory reads and writes. The kernel futex operations are protected .\" by spinlocks, which ensure that all operations are serialized .\" with respect to one another. .\" .\" This is a lot to attempt to define in this document. Perhaps a .\" reference to linux/Documentation/memory-barriers.txt as a footnote .\" would be sufficient? Or perhaps for this manual, "serialized" would .\" be sufficient, with a footnote regarding "totally ordered" and a .\" pointer to the memory-barrier documentation? Thus, the futex word is used to connect the synchronization in user space with the implementation of blocking by the kernel. Analogously to an atomic compare-and-exchange operation that potentially changes shared memory, blocking via a futex is an atomic compare-and-block operation. .\" FIXME(Torvald Riegel): .\" Eventually we want to have some text in NOTES to satisfy .\" the reference in the following sentence .\" See NOTES for a detailed specification of .\" the synchronization semantics. .PP One use of futexes is for implementing locks. The state of the lock (i.e., acquired or not acquired) can be represented as an atomically accessed flag in shared memory. In the uncontended case, a thread can access or modify the lock state with atomic instructions, for example atomically changing it from not acquired to acquired using an atomic compare-and-exchange instruction. (Such instructions are performed entirely in user mode, and the kernel maintains no information about the lock state.) On the other hand, a thread may be unable to acquire a lock because it is already acquired by another thread. It then may pass the lock's flag as a futex word and the value representing the acquired state as the expected value to a .BR futex () wait operation. This .BR futex () operation will block if and only if the lock is still acquired (i.e., the value in the futex word still matches the "acquired state"). When releasing the lock, a thread has to first reset the lock state to not acquired and then execute a futex operation that wakes threads blocked on the lock flag used as a futex word (this can be further optimized to avoid unnecessary wake-ups). See .BR futex (7) for more detail on how to use futexes. .PP Besides the basic wait and wake-up futex functionality, there are further futex operations aimed at supporting more complex use cases. .PP Note that no explicit initialization or destruction is necessary to use futexes; the kernel maintains a futex (i.e., the kernel-internal implementation artifact) only while operations such as .BR FUTEX_WAIT , described below, are being performed on a particular futex word. .\" .SS Arguments The .I uaddr argument points to the futex word. On all platforms, futexes are four-byte integers that must be aligned on a four-byte boundary. The operation to perform on the futex is specified in the .I futex_op argument; .IR val is a value whose meaning and purpose depends on .IR futex_op . .PP The remaining arguments .RI ( timeout , .IR uaddr2 , and .IR val3 ) are required only for certain of the futex operations described below. Where one of these arguments is not required, it is ignored. .PP For several blocking operations, the .I timeout argument is a pointer to a .IR timespec structure that specifies a timeout for the operation. However, notwithstanding the prototype shown above, for some operations, the least significant four bytes of this argument are instead used as an integer whose meaning is determined by the operation. For these operations, the kernel casts the .I timeout value first to .IR "unsigned long", then to .IR uint32_t , and in the remainder of this page, this argument is referred to as .I val2 when interpreted in this fashion. .PP Where it is required, the .IR uaddr2 argument is a pointer to a second futex word that is employed by the operation. .PP The interpretation of the final integer argument, .IR val3 , depends on the operation. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .SS Futex operations The .I futex_op argument consists of two parts: a command that specifies the operation to be performed, bitwise ORed with zero or more options that modify the behaviour of the operation. The options that may be included in .I futex_op are as follows: .TP .BR FUTEX_PRIVATE_FLAG " (since Linux 2.6.22)" .\" commit 34f01cc1f512fa783302982776895c73714ebbc2 This option bit can be employed with all futex operations. It tells the kernel that the futex is process-private and not shared with another process (i.e., it is being used for synchronization only between threads of the same process). This allows the kernel to make some additional performance optimizations. .\" I.e., It allows the kernel choose the fast path for validating .\" the user-space address and avoids expensive VMA lookups, .\" taking reference counts on file backing store, and so on. .IP As a convenience, .IR defines a set of constants with the suffix .BR _PRIVATE that are equivalents of all of the operations listed below, .\" except the obsolete FUTEX_FD, for which the "private" flag was .\" meaningless but with the .BR FUTEX_PRIVATE_FLAG ORed into the constant value. Thus, there are .BR FUTEX_WAIT_PRIVATE , .BR FUTEX_WAKE_PRIVATE , and so on. .TP .BR FUTEX_CLOCK_REALTIME " (since Linux 2.6.28)" .\" commit 1acdac104668a0834cfa267de9946fac7764d486 This option bit can be employed only with the .BR FUTEX_WAIT_BITSET , .BR FUTEX_WAIT_REQUEUE_PI , and (since Linux 4.5) .\" commit 337f13046ff03717a9e99675284a817527440a49 .BR FUTEX_WAIT operations. .IP If this option is set, the kernel measures the .I timeout against the .BR CLOCK_REALTIME clock. .IP If this option is not set, the kernel measures the .I timeout against the .BR CLOCK_MONOTONIC clock. .PP The operation specified in .I futex_op is one of the following: .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_WAIT " (since Linux 2.6.0)" .\" Strictly speaking, since some time in 2.5.x This operation tests that the value at the futex word pointed to by the address .I uaddr still contains the expected value .IR val , and if so, then sleeps waiting for a .B FUTEX_WAKE operation on the futex word. The load of the value of the futex word is an atomic memory access (i.e., using atomic machine instructions of the respective architecture). This load, the comparison with the expected value, and starting to sleep are performed atomically .\" FIXME: Torvald, I think we may need to add some explanation of .\" "totally ordered" here. and totally ordered with respect to other futex operations on the same futex word. If the thread starts to sleep, it is considered a waiter on this futex word. If the futex value does not match .IR val , then the call fails immediately with the error .BR EAGAIN . .IP The purpose of the comparison with the expected value is to prevent lost wake-ups. If another thread changed the value of the futex word after the calling thread decided to block based on the prior value, and if the other thread executed a .BR FUTEX_WAKE operation (or similar wake-up) after the value change and before this .BR FUTEX_WAIT operation, then the calling thread will observe the value change and will not start to sleep. .IP If the .I timeout is not NULL, the structure it points to specifies a timeout for the wait. (This interval will be rounded up to the system clock granularity, and is guaranteed not to expire early.) The timeout is by default measured according to the .BR CLOCK_MONOTONIC clock, but, since Linux 4.5, the .BR CLOCK_REALTIME clock can be selected by specifying .BR FUTEX_CLOCK_REALTIME in .IR futex_op . If .I timeout is NULL, the call blocks indefinitely. .IP .IR Note : for .BR FUTEX_WAIT , .IR timeout is interpreted as a .IR relative value. This differs from other futex operations, where .I timeout is interpreted as an absolute value. To obtain the equivalent of .BR FUTEX_WAIT with an absolute timeout, employ .BR FUTEX_WAIT_BITSET with .IR val3 specified as .BR FUTEX_BITSET_MATCH_ANY . .IP The arguments .I uaddr2 and .I val3 are ignored. .\" FIXME . (Torvald) I think we should remove this. Or maybe adapt to a .\" different example. .\" .\" For .\" .BR futex (7), .\" this call is executed if decrementing the count gave a negative value .\" (indicating contention), .\" and will sleep until another process or thread releases .\" the futex and executes the .\" .B FUTEX_WAKE .\" operation. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_WAKE " (since Linux 2.6.0)" .\" Strictly speaking, since Linux 2.5.x This operation wakes at most .I val of the waiters that are waiting (e.g., inside .BR FUTEX_WAIT ) on the futex word at the address .IR uaddr . Most commonly, .I val is specified as either 1 (wake up a single waiter) or .BR INT_MAX (wake up all waiters). No guarantee is provided about which waiters are awoken (e.g., a waiter with a higher scheduling priority is not guaranteed to be awoken in preference to a waiter with a lower priority). .IP The arguments .IR timeout , .IR uaddr2 , and .I val3 are ignored. .\" FIXME . (Torvald) I think we should remove this. Or maybe adapt to .\" a different example. .\" .\" For .\" .BR futex (7), .\" this is executed if incrementing the count showed that .\" there were waiters, .\" once the futex value has been set to 1 .\" (indicating that it is available). .\" .\" How does "incrementing the count show that there were waiters"? .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_FD " (from Linux 2.6.0 up to and including Linux 2.6.25)" .\" Strictly speaking, from Linux 2.5.x to 2.6.25 This operation creates a file descriptor that is associated with the futex at .IR uaddr . The caller must close the returned file descriptor after use. When another process or thread performs a .BR FUTEX_WAKE on the futex word, the file descriptor indicates as being readable with .BR select (2), .BR poll (2), and .BR epoll (7) .IP The file descriptor can be used to obtain asynchronous notifications: if .I val is nonzero, then, when another process or thread executes a .BR FUTEX_WAKE , the caller will receive the signal number that was passed in .IR val . .IP The arguments .IR timeout , .IR uaddr2 , and .I val3 are ignored. .IP Because it was inherently racy, .B FUTEX_FD has been removed .\" commit 82af7aca56c67061420d618cc5a30f0fd4106b80 from Linux 2.6.26 onward. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_REQUEUE " (since Linux 2.6.0)" This operation performs the same task as .BR FUTEX_CMP_REQUEUE (see below), except that no check is made using the value in .IR val3 . (The argument .I val3 is ignored.) .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_CMP_REQUEUE " (since Linux 2.6.7)" This operation first checks whether the location .I uaddr still contains the value .IR val3 . If not, the operation fails with the error .BR EAGAIN . Otherwise, the operation wakes up a maximum of .I val waiters that are waiting on the futex at .IR uaddr . If there are more than .I val waiters, then the remaining waiters are removed from the wait queue of the source futex at .I uaddr and added to the wait queue of the target futex at .IR uaddr2 . The .I val2 argument specifies an upper limit on the number of waiters that are requeued to the futex at .IR uaddr2 . .IP .\" FIXME(Torvald) Is the following correct? Or is just the decision .\" which threads to wake or requeue part of the atomic operation? The load from .I uaddr is an atomic memory access (i.e., using atomic machine instructions of the respective architecture). This load, the comparison with .IR val3 , and the requeueing of any waiters are performed atomically and totally ordered with respect to other operations on the same futex word. .\" Notes from a f2f conversation with Thomas Gleixner (Aug 2015): ### .\" The operation is serialized with respect to operations on both .\" source and target futex. No other waiter can enqueue itself .\" for waiting and no other waiter can dequeue itself because of .\" a timeout or signal. .IP Typical values to specify for .I val are 0 or 1. (Specifying .BR INT_MAX is not useful, because it would make the .BR FUTEX_CMP_REQUEUE operation equivalent to .BR FUTEX_WAKE .) The limit value specified via .I val2 is typically either 1 or .BR INT_MAX . (Specifying the argument as 0 is not useful, because it would make the .BR FUTEX_CMP_REQUEUE operation equivalent to .BR FUTEX_WAIT .) .IP The .B FUTEX_CMP_REQUEUE operation was added as a replacement for the earlier .BR FUTEX_REQUEUE . The difference is that the check of the value at .I uaddr can be used to ensure that requeueing happens only under certain conditions, which allows race conditions to be avoided in certain use cases. .\" But, as Rich Felker points out, there remain valid use cases for .\" FUTEX_REQUEUE, for example, when the calling thread is requeuing .\" the target(s) to a lock that the calling thread owns .\" From: Rich Felker .\" Date: Wed, 29 Oct 2014 22:43:17 -0400 .\" To: Darren Hart .\" CC: libc-alpha@sourceware.org, ... .\" Subject: Re: Add futex wrapper to glibc? .IP Both .BR FUTEX_REQUEUE and .BR FUTEX_CMP_REQUEUE can be used to avoid "thundering herd" wake-ups that could occur when using .B FUTEX_WAKE in cases where all of the waiters that are woken need to acquire another futex. Consider the following scenario, where multiple waiter threads are waiting on B, a wait queue implemented using a futex: .IP .in +4n .EX lock(A) while (!check_value(V)) { unlock(A); block_on(B); lock(A); }; unlock(A); .EE .in .IP If a waker thread used .BR FUTEX_WAKE , then all waiters waiting on B would be woken up, and they would all try to acquire lock A. However, waking all of the threads in this manner would be pointless because all except one of the threads would immediately block on lock A again. By contrast, a requeue operation wakes just one waiter and moves the other waiters to lock A, and when the woken waiter unlocks A then the next waiter can proceed. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_WAKE_OP " (since Linux 2.6.14)" .\" commit 4732efbeb997189d9f9b04708dc26bf8613ed721 .\" Author: Jakub Jelinek .\" Date: Tue Sep 6 15:16:25 2005 -0700 .\" FIXME. (Torvald) The glibc condvar implementation is currently being .\" revised (e.g., to not use an internal lock anymore). .\" It is probably more future-proof to remove this paragraph. .\" [Torvald, do you have an update here?] This operation was added to support some user-space use cases where more than one futex must be handled at the same time. The most notable example is the implementation of .BR pthread_cond_signal (3), which requires operations on two futexes, the one used to implement the mutex and the one used in the implementation of the wait queue associated with the condition variable. .BR FUTEX_WAKE_OP allows such cases to be implemented without leading to high rates of contention and context switching. .IP The .BR FUTEX_WAKE_OP operation is equivalent to executing the following code atomically and totally ordered with respect to other futex operations on any of the two supplied futex words: .IP .in +4n .EX uint32_t oldval = *(uint32_t *) uaddr2; *(uint32_t *) uaddr2 = oldval \fIop\fP \fIoparg\fP; futex(uaddr, FUTEX_WAKE, val, 0, 0, 0); if (oldval \fIcmp\fP \fIcmparg\fP) futex(uaddr2, FUTEX_WAKE, val2, 0, 0, 0); .EE .in .IP In other words, .BR FUTEX_WAKE_OP does the following: .RS .IP * 3 saves the original value of the futex word at .IR uaddr2 and performs an operation to modify the value of the futex at .IR uaddr2 ; this is an atomic read-modify-write memory access (i.e., using atomic machine instructions of the respective architecture) .IP * wakes up a maximum of .I val waiters on the futex for the futex word at .IR uaddr ; and .IP * dependent on the results of a test of the original value of the futex word at .IR uaddr2 , wakes up a maximum of .I val2 waiters on the futex for the futex word at .IR uaddr2 . .RE .IP The operation and comparison that are to be performed are encoded in the bits of the argument .IR val3 . Pictorially, the encoding is: .IP .in +4n .EX +---+---+-----------+-----------+ |op |cmp| oparg | cmparg | +---+---+-----------+-----------+ 4 4 12 12 <== # of bits .EE .in .IP Expressed in code, the encoding is: .IP .in +4n .EX #define FUTEX_OP(op, oparg, cmp, cmparg) \e (((op & 0xf) << 28) | \e ((cmp & 0xf) << 24) | \e ((oparg & 0xfff) << 12) | \e (cmparg & 0xfff)) .EE .in .IP In the above, .I op and .I cmp are each one of the codes listed below. The .I oparg and .I cmparg components are literal numeric values, except as noted below. .IP The .I op component has one of the following values: .IP .in +4n .EX FUTEX_OP_SET 0 /* uaddr2 = oparg; */ FUTEX_OP_ADD 1 /* uaddr2 += oparg; */ FUTEX_OP_OR 2 /* uaddr2 |= oparg; */ FUTEX_OP_ANDN 3 /* uaddr2 &= \(tioparg; */ FUTEX_OP_XOR 4 /* uaddr2 \(ha= oparg; */ .EE .in .IP In addition, bitwise ORing the following value into .I op causes .IR "(1\ <<\ oparg)" to be used as the operand: .IP .in +4n .EX FUTEX_OP_ARG_SHIFT 8 /* Use (1 << oparg) as operand */ .EE .in .IP The .I cmp field is one of the following: .IP .in +4n .EX FUTEX_OP_CMP_EQ 0 /* if (oldval == cmparg) wake */ FUTEX_OP_CMP_NE 1 /* if (oldval != cmparg) wake */ FUTEX_OP_CMP_LT 2 /* if (oldval < cmparg) wake */ FUTEX_OP_CMP_LE 3 /* if (oldval <= cmparg) wake */ FUTEX_OP_CMP_GT 4 /* if (oldval > cmparg) wake */ FUTEX_OP_CMP_GE 5 /* if (oldval >= cmparg) wake */ .EE .in .IP The return value of .BR FUTEX_WAKE_OP is the sum of the number of waiters woken on the futex .IR uaddr plus the number of waiters woken on the futex .IR uaddr2 . .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_WAIT_BITSET " (since Linux 2.6.25)" .\" commit cd689985cf49f6ff5c8eddc48d98b9d581d9475d This operation is like .BR FUTEX_WAIT except that .I val3 is used to provide a 32-bit bit mask to the kernel. This bit mask, in which at least one bit must be set, is stored in the kernel-internal state of the waiter. See the description of .BR FUTEX_WAKE_BITSET for further details. .IP If .I timeout is not NULL, the structure it points to specifies an absolute timeout for the wait operation. If .I timeout is NULL, the operation can block indefinitely. .IP The .I uaddr2 argument is ignored. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_WAKE_BITSET " (since Linux 2.6.25)" .\" commit cd689985cf49f6ff5c8eddc48d98b9d581d9475d This operation is the same as .BR FUTEX_WAKE except that the .I val3 argument is used to provide a 32-bit bit mask to the kernel. This bit mask, in which at least one bit must be set, is used to select which waiters should be woken up. The selection is done by a bitwise AND of the "wake" bit mask (i.e., the value in .IR val3 ) and the bit mask which is stored in the kernel-internal state of the waiter (the "wait" bit mask that is set using .BR FUTEX_WAIT_BITSET ). All of the waiters for which the result of the AND is nonzero are woken up; the remaining waiters are left sleeping. .IP The effect of .BR FUTEX_WAIT_BITSET and .BR FUTEX_WAKE_BITSET is to allow selective wake-ups among multiple waiters that are blocked on the same futex. However, note that, depending on the use case, employing this bit-mask multiplexing feature on a futex can be less efficient than simply using multiple futexes, because employing bit-mask multiplexing requires the kernel to check all waiters on a futex, including those that are not interested in being woken up (i.e., they do not have the relevant bit set in their "wait" bit mask). .\" According to http://locklessinc.com/articles/futex_cheat_sheet/: .\" .\" "The original reason for the addition of these extensions .\" was to improve the performance of pthread read-write locks .\" in glibc. However, the pthreads library no longer uses the .\" same locking algorithm, and these extensions are not used .\" without the bitset parameter being all ones. .\" .\" The page goes on to note that the FUTEX_WAIT_BITSET operation .\" is nevertheless used (with a bit mask of all ones) in order to .\" obtain the absolute timeout functionality that is useful .\" for efficiently implementing Pthreads APIs (which use absolute .\" timeouts); FUTEX_WAIT provides only relative timeouts. .IP The constant .BR FUTEX_BITSET_MATCH_ANY , which corresponds to all 32 bits set in the bit mask, can be used as the .I val3 argument for .BR FUTEX_WAIT_BITSET and .BR FUTEX_WAKE_BITSET . Other than differences in the handling of the .I timeout argument, the .BR FUTEX_WAIT operation is equivalent to .BR FUTEX_WAIT_BITSET with .IR val3 specified as .BR FUTEX_BITSET_MATCH_ANY ; that is, allow a wake-up by any waker. The .BR FUTEX_WAKE operation is equivalent to .BR FUTEX_WAKE_BITSET with .IR val3 specified as .BR FUTEX_BITSET_MATCH_ANY ; that is, wake up any waiter(s). .IP The .I uaddr2 and .I timeout arguments are ignored. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .SS Priority-inheritance futexes Linux supports priority-inheritance (PI) futexes in order to handle priority-inversion problems that can be encountered with normal futex locks. Priority inversion is the problem that occurs when a high-priority task is blocked waiting to acquire a lock held by a low-priority task, while tasks at an intermediate priority continuously preempt the low-priority task from the CPU. Consequently, the low-priority task makes no progress toward releasing the lock, and the high-priority task remains blocked. .PP Priority inheritance is a mechanism for dealing with the priority-inversion problem. With this mechanism, when a high-priority task becomes blocked by a lock held by a low-priority task, the priority of the low-priority task is temporarily raised to that of the high-priority task, so that it is not preempted by any intermediate level tasks, and can thus make progress toward releasing the lock. To be effective, priority inheritance must be transitive, meaning that if a high-priority task blocks on a lock held by a lower-priority task that is itself blocked by a lock held by another intermediate-priority task (and so on, for chains of arbitrary length), then both of those tasks (or more generally, all of the tasks in a lock chain) have their priorities raised to be the same as the high-priority task. .PP From a user-space perspective, what makes a futex PI-aware is a policy agreement (described below) between user space and the kernel about the value of the futex word, coupled with the use of the PI-futex operations described below. (Unlike the other futex operations described above, the PI-futex operations are designed for the implementation of very specific IPC mechanisms.) .\" .\" Quoting Darren Hart: .\" These opcodes paired with the PI futex value policy (described below) .\" defines a "futex" as PI aware. These were created very specifically .\" in support of PI pthread_mutexes, so it makes a lot more sense to .\" talk about a PI aware pthread_mutex, than a PI aware futex, since .\" there is a lot of policy and scaffolding that has to be built up .\" around it to use it properly (this is what a PI pthread_mutex is). .PP .\" mtk: The following text is drawn from the Hart/Guniguntala paper .\" (listed in SEE ALSO), but I have reworded some pieces .\" significantly. .\" The PI-futex operations described below differ from the other futex operations in that they impose policy on the use of the value of the futex word: .IP * 3 If the lock is not acquired, the futex word's value shall be 0. .IP * If the lock is acquired, the futex word's value shall be the thread ID (TID; see .BR gettid (2)) of the owning thread. .IP * If the lock is owned and there are threads contending for the lock, then the .B FUTEX_WAITERS bit shall be set in the futex word's value; in other words, this value is: .IP FUTEX_WAITERS | TID .IP (Note that is invalid for a PI futex word to have no owner and .BR FUTEX_WAITERS set.) .PP With this policy in place, a user-space application can acquire an unacquired lock or release a lock using atomic instructions executed in user mode (e.g., a compare-and-swap operation such as .I cmpxchg on the x86 architecture). Acquiring a lock simply consists of using compare-and-swap to atomically set the futex word's value to the caller's TID if its previous value was 0. Releasing a lock requires using compare-and-swap to set the futex word's value to 0 if the previous value was the expected TID. .PP If a futex is already acquired (i.e., has a nonzero value), waiters must employ the .B FUTEX_LOCK_PI operation to acquire the lock. If other threads are waiting for the lock, then the .B FUTEX_WAITERS bit is set in the futex value; in this case, the lock owner must employ the .B FUTEX_UNLOCK_PI operation to release the lock. .PP In the cases where callers are forced into the kernel (i.e., required to perform a .BR futex () call), they then deal directly with a so-called RT-mutex, a kernel locking mechanism which implements the required priority-inheritance semantics. After the RT-mutex is acquired, the futex value is updated accordingly, before the calling thread returns to user space. .PP It is important to note .\" tglx (July 2015): .\" If there are multiple waiters on a pi futex then a wake pi operation .\" will wake the first waiter and hand over the lock to this waiter. This .\" includes handing over the rtmutex which represents the futex in the .\" kernel. The strict requirement is that the futex owner and the rtmutex .\" owner must be the same, except for the update period which is .\" serialized by the futex internal locking. That means the kernel must .\" update the user-space value prior to returning to user space that the kernel will update the futex word's value prior to returning to user space. (This prevents the possibility of the futex word's value ending up in an invalid state, such as having an owner but the value being 0, or having waiters but not having the .B FUTEX_WAITERS bit set.) .PP If a futex has an associated RT-mutex in the kernel (i.e., there are blocked waiters) and the owner of the futex/RT-mutex dies unexpectedly, then the kernel cleans up the RT-mutex and hands it over to the next waiter. This in turn requires that the user-space value is updated accordingly. To indicate that this is required, the kernel sets the .B FUTEX_OWNER_DIED bit in the futex word along with the thread ID of the new owner. User space can detect this situation via the presence of the .B FUTEX_OWNER_DIED bit and is then responsible for cleaning up the stale state left over by the dead owner. .\" tglx (July 2015): .\" The FUTEX_OWNER_DIED bit can also be set on uncontended futexes, where .\" the kernel has no state associated. This happens via the robust futex .\" mechanism. In that case the futex value will be set to .\" FUTEX_OWNER_DIED. The robust futex mechanism is also available for non .\" PI futexes. .PP PI futexes are operated on by specifying one of the values listed below in .IR futex_op . Note that the PI futex operations must be used as paired operations and are subject to some additional requirements: .IP * 3 .B FUTEX_LOCK_PI and .B FUTEX_TRYLOCK_PI pair with .BR FUTEX_UNLOCK_PI . .B FUTEX_UNLOCK_PI must be called only on a futex owned by the calling thread, as defined by the value policy, otherwise the error .B EPERM results. .IP * .B FUTEX_WAIT_REQUEUE_PI pairs with .BR FUTEX_CMP_REQUEUE_PI . This must be performed from a non-PI futex to a distinct PI futex (or the error .B EINVAL results). Additionally, .I val (the number of waiters to be woken) must be 1 (or the error .B EINVAL results). .PP The PI futex operations are as follows: .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_LOCK_PI " (since Linux 2.6.18)" .\" commit c87e2837be82df479a6bae9f155c43516d2feebc This operation is used after an attempt to acquire the lock via an atomic user-mode instruction failed because the futex word has a nonzero value\(emspecifically, because it contained the (PID-namespace-specific) TID of the lock owner. .IP The operation checks the value of the futex word at the address .IR uaddr . If the value is 0, then the kernel tries to atomically set the futex value to the caller's TID. If the futex word's value is nonzero, the kernel atomically sets the .B FUTEX_WAITERS bit, which signals the futex owner that it cannot unlock the futex in user space atomically by setting the futex value to 0. .\" tglx (July 2015): .\" The operation here is similar to the FUTEX_WAIT logic. When the user .\" space atomic acquire does not succeed because the futex value was non .\" zero, then the waiter goes into the kernel, takes the kernel internal .\" lock and retries the acquisition under the lock. If the acquisition .\" does not succeed either, then it sets the FUTEX_WAITERS bit, to signal .\" the lock owner that it needs to go into the kernel. Here is the pseudo .\" code: .\" .\" lock(kernel_lock); .\" retry: .\" .\" /* .\" * Owner might have unlocked in userspace before we .\" * were able to set the waiter bit. .\" */ .\" if (atomic_acquire(futex) == SUCCESS) { .\" unlock(kernel_lock()); .\" return 0; .\" } .\" .\" /* .\" * Owner might have unlocked after the above atomic_acquire() .\" * attempt. .\" */ .\" if (atomic_set_waiters_bit(futex) != SUCCESS) .\" goto retry; .\" .\" queue_waiter(); .\" unlock(kernel_lock); .\" block(); .\" After that, the kernel: .RS .IP 1. 3 Tries to find the thread which is associated with the owner TID. .IP 2. Creates or reuses kernel state on behalf of the owner. (If this is the first waiter, there is no kernel state for this futex, so kernel state is created by locking the RT-mutex and the futex owner is made the owner of the RT-mutex. If there are existing waiters, then the existing state is reused.) .IP 3. Attaches the waiter to the futex (i.e., the waiter is enqueued on the RT-mutex waiter list). .RE .IP If more than one waiter exists, the enqueueing of the waiter is in descending priority order. (For information on priority ordering, see the discussion of the .BR SCHED_DEADLINE , .BR SCHED_FIFO , and .BR SCHED_RR scheduling policies in .BR sched (7).) The owner inherits either the waiter's CPU bandwidth (if the waiter is scheduled under the .BR SCHED_DEADLINE policy) or the waiter's priority (if the waiter is scheduled under the .BR SCHED_RR or .BR SCHED_FIFO policy). .\" August 2015: .\" mtk: If the realm is restricted purely to SCHED_OTHER (SCHED_NORMAL) .\" processes, does the nice value come into play also? .\" .\" tglx: No. SCHED_OTHER/NORMAL tasks are handled in FIFO order This inheritance follows the lock chain in the case of nested locking .\" (i.e., task 1 blocks on lock A, held by task 2, .\" while task 2 blocks on lock B, held by task 3) and performs deadlock detection. .IP The .I timeout argument provides a timeout for the lock attempt. If .I timeout is not NULL, the structure it points to specifies an absolute timeout, measured against the .BR CLOCK_REALTIME clock. .\" 2016-07-07 response from Thomas Gleixner on LKML: .\" From: Thomas Gleixner .\" Date: 6 July 2016 at 20:57 .\" Subject: Re: futex: Allow FUTEX_CLOCK_REALTIME with FUTEX_WAIT op .\" .\" On Thu, 23 Jun 2016, Michael Kerrisk (man-pages) wrote: .\" > On 06/23/2016 08:28 PM, Darren Hart wrote: .\" > > And as a follow-on, what is the reason for FUTEX_LOCK_PI only using .\" > > CLOCK_REALTIME? It seems reasonable to me that a user may want to wait a .\" > > specific amount of time, regardless of wall time. .\" > .\" > Yes, that's another weird inconsistency. .\" .\" The reason is that phtread_mutex_timedlock() uses absolute timeouts based on .\" CLOCK_REALTIME. glibc folks asked to make that the default behaviour back .\" then when we added LOCK_PI. If .I timeout is NULL, the operation will block indefinitely. .IP The .IR uaddr2 , .IR val , and .IR val3 arguments are ignored. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_TRYLOCK_PI " (since Linux 2.6.18)" .\" commit c87e2837be82df479a6bae9f155c43516d2feebc This operation tries to acquire the lock at .IR uaddr . It is invoked when a user-space atomic acquire did not succeed because the futex word was not 0. .IP Because the kernel has access to more state information than user space, acquisition of the lock might succeed if performed by the kernel in cases where the futex word (i.e., the state information accessible to use-space) contains stale state .RB ( FUTEX_WAITERS and/or .BR FUTEX_OWNER_DIED ). This can happen when the owner of the futex died. User space cannot handle this condition in a race-free manner, but the kernel can fix this up and acquire the futex. .\" Paraphrasing a f2f conversation with Thomas Gleixner about the .\" above point (Aug 2015): ### .\" There is a rare possibility of a race condition involving an .\" uncontended futex with no owner, but with waiters. The .\" kernel-user-space contract is that if a futex is nonzero, you must .\" go into kernel. The futex was owned by a task, and that task dies .\" but there are no waiters, so the futex value is non zero. .\" Therefore, the next locker has to go into the kernel, .\" so that the kernel has a chance to clean up. (CMXCH on zero .\" in user space would fail, so kernel has to clean up.) .\" Darren Hart (Oct 2015): .\" The trylock in the kernel has more state, so it can independently .\" verify the flags that userspace must trust implicitly. .IP The .IR uaddr2 , .IR val , .IR timeout , and .IR val3 arguments are ignored. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_UNLOCK_PI " (since Linux 2.6.18)" .\" commit c87e2837be82df479a6bae9f155c43516d2feebc This operation wakes the top priority waiter that is waiting in .B FUTEX_LOCK_PI on the futex address provided by the .I uaddr argument. .IP This is called when the user-space value at .I uaddr cannot be changed atomically from a TID (of the owner) to 0. .IP The .IR uaddr2 , .IR val , .IR timeout , and .IR val3 arguments are ignored. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_CMP_REQUEUE_PI " (since Linux 2.6.31)" .\" commit 52400ba946759af28442dee6265c5c0180ac7122 This operation is a PI-aware variant of .BR FUTEX_CMP_REQUEUE . It requeues waiters that are blocked via .B FUTEX_WAIT_REQUEUE_PI on .I uaddr from a non-PI source futex .RI ( uaddr ) to a PI target futex .RI ( uaddr2 ). .IP As with .BR FUTEX_CMP_REQUEUE , this operation wakes up a maximum of .I val waiters that are waiting on the futex at .IR uaddr . However, for .BR FUTEX_CMP_REQUEUE_PI , .I val is required to be 1 (since the main point is to avoid a thundering herd). The remaining waiters are removed from the wait queue of the source futex at .I uaddr and added to the wait queue of the target futex at .IR uaddr2 . .IP The .I val2 .\" val2 is the cap on the number of requeued waiters. .\" In the glibc pthread_cond_broadcast() implementation, this argument .\" is specified as INT_MAX, and for pthread_cond_signal() it is 0. and .I val3 arguments serve the same purposes as for .BR FUTEX_CMP_REQUEUE . .\" .\" The page at http://locklessinc.com/articles/futex_cheat_sheet/ .\" notes that "priority-inheritance Futex to priority-inheritance .\" Futex requeues are currently unsupported". However, probably .\" the page does not need to say nothing about this, since .\" Thomas Gleixner commented (July 2015): "they never will be .\" supported because they make no sense at all" .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_WAIT_REQUEUE_PI " (since Linux 2.6.31)" .\" commit 52400ba946759af28442dee6265c5c0180ac7122 .\" Wait on a non-PI futex at .I uaddr and potentially be requeued (via a .BR FUTEX_CMP_REQUEUE_PI operation in another task) onto a PI futex at .IR uaddr2 . The wait operation on .I uaddr is the same as for .BR FUTEX_WAIT . .IP The waiter can be removed from the wait on .I uaddr without requeueing on .IR uaddr2 via a .BR FUTEX_WAKE operation in another task. In this case, the .BR FUTEX_WAIT_REQUEUE_PI operation fails with the error .BR EAGAIN . .IP If .I timeout is not NULL, the structure it points to specifies an absolute timeout for the wait operation. If .I timeout is NULL, the operation can block indefinitely. .IP The .I val3 argument is ignored. .IP The .BR FUTEX_WAIT_REQUEUE_PI and .BR FUTEX_CMP_REQUEUE_PI were added to support a fairly specific use case: support for priority-inheritance-aware POSIX threads condition variables. The idea is that these operations should always be paired, in order to ensure that user space and the kernel remain in sync. Thus, in the .BR FUTEX_WAIT_REQUEUE_PI operation, the user-space application pre-specifies the target of the requeue that takes place in the .BR FUTEX_CMP_REQUEUE_PI operation. .\" .\" Darren Hart notes that a patch to allow glibc to fully support .\" PI-aware pthreads condition variables has not yet been accepted into .\" glibc. The story is complex, and can be found at .\" https://sourceware.org/bugzilla/show_bug.cgi?id=11588 .\" Darren notes that in the meantime, the patch is shipped with various .\" PREEMPT_RT-enabled Linux systems. .\" .\" Related to the preceding, Darren proposed that somewhere, man-pages .\" should document the following point: .\" .\" While the Linux kernel, since 2.6.31, supports requeueing of .\" priority-inheritance (PI) aware mutexes via the .\" FUTEX_WAIT_REQUEUE_PI and FUTEX_CMP_REQUEUE_PI futex operations, .\" the glibc implementation does not yet take full advantage of this. .\" Specifically, the condvar internal data lock remains a non-PI aware .\" mutex, regardless of the type of the pthread_mutex associated with .\" the condvar. This can lead to an unbounded priority inversion on .\" the internal data lock even when associating a PI aware .\" pthread_mutex with a condvar during a pthread_cond*_wait .\" operation. For this reason, it is not recommended to rely on .\" priority inheritance when using pthread condition variables. .\" .\" The problem is that the obvious location for this text is .\" the pthread_cond*wait(3) man page. However, such a man page .\" does not currently exist. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .SH RETURN VALUE In the event of an error (and assuming that .BR futex () was invoked via .BR syscall (2)), all operations return \-1 and set .I errno to indicate the cause of the error. .PP The return value on success depends on the operation, as described in the following list: .TP .B FUTEX_WAIT Returns 0 if the caller was woken up. Note that a wake-up can also be caused by common futex usage patterns in unrelated code that happened to have previously used the futex word's memory location (e.g., typical futex-based implementations of Pthreads mutexes can cause this under some conditions). Therefore, callers should always conservatively assume that a return value of 0 can mean a spurious wake-up, and use the futex word's value (i.e., the user-space synchronization scheme) to decide whether to continue to block or not. .TP .B FUTEX_WAKE Returns the number of waiters that were woken up. .TP .B FUTEX_FD Returns the new file descriptor associated with the futex. .TP .B FUTEX_REQUEUE Returns the number of waiters that were woken up. .TP .B FUTEX_CMP_REQUEUE Returns the total number of waiters that were woken up or requeued to the futex for the futex word at .IR uaddr2 . If this value is greater than .IR val , then the difference is the number of waiters requeued to the futex for the futex word at .IR uaddr2 . .TP .B FUTEX_WAKE_OP Returns the total number of waiters that were woken up. This is the sum of the woken waiters on the two futexes for the futex words at .I uaddr and .IR uaddr2 . .TP .B FUTEX_WAIT_BITSET Returns 0 if the caller was woken up. See .B FUTEX_WAIT for how to interpret this correctly in practice. .TP .B FUTEX_WAKE_BITSET Returns the number of waiters that were woken up. .TP .B FUTEX_LOCK_PI Returns 0 if the futex was successfully locked. .TP .B FUTEX_TRYLOCK_PI Returns 0 if the futex was successfully locked. .TP .B FUTEX_UNLOCK_PI Returns 0 if the futex was successfully unlocked. .TP .B FUTEX_CMP_REQUEUE_PI Returns the total number of waiters that were woken up or requeued to the futex for the futex word at .IR uaddr2 . If this value is greater than .IR val , then difference is the number of waiters requeued to the futex for the futex word at .IR uaddr2 . .TP .B FUTEX_WAIT_REQUEUE_PI Returns 0 if the caller was successfully requeued to the futex for the futex word at .IR uaddr2 . .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .SH ERRORS .TP .B EACCES No read access to the memory of a futex word. .TP .B EAGAIN .RB ( FUTEX_WAIT , .BR FUTEX_WAIT_BITSET , .BR FUTEX_WAIT_REQUEUE_PI ) The value pointed to by .I uaddr was not equal to the expected value .I val at the time of the call. .IP .BR Note : on Linux, the symbolic names .B EAGAIN and .B EWOULDBLOCK (both of which appear in different parts of the kernel futex code) have the same value. .TP .B EAGAIN .RB ( FUTEX_CMP_REQUEUE , .BR FUTEX_CMP_REQUEUE_PI ) The value pointed to by .I uaddr is not equal to the expected value .IR val3 . .TP .BR EAGAIN .RB ( FUTEX_LOCK_PI , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_CMP_REQUEUE_PI ) The futex owner thread ID of .I uaddr (for .BR FUTEX_CMP_REQUEUE_PI : .IR uaddr2 ) is about to exit, but has not yet handled the internal state cleanup. Try again. .TP .BR EDEADLK .RB ( FUTEX_LOCK_PI , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_CMP_REQUEUE_PI ) The futex word at .I uaddr is already locked by the caller. .TP .BR EDEADLK .\" FIXME . I see that kernel/locking/rtmutex.c uses EDEADLK in some .\" places, and EDEADLOCK in others. On almost all architectures .\" these constants are synonymous. Is there a reason that both .\" names are used? .\" .\" tglx (July 2015): "No. We should probably fix that." .\" .RB ( FUTEX_CMP_REQUEUE_PI ) While requeueing a waiter to the PI futex for the futex word at .IR uaddr2 , the kernel detected a deadlock. .TP .B EFAULT A required pointer argument (i.e., .IR uaddr , .IR uaddr2 , or .IR timeout ) did not point to a valid user-space address. .TP .B EINTR A .B FUTEX_WAIT or .B FUTEX_WAIT_BITSET operation was interrupted by a signal (see .BR signal (7)). In kernels before Linux 2.6.22, this error could also be returned for a spurious wakeup; since Linux 2.6.22, this no longer happens. .TP .B EINVAL The operation in .IR futex_op is one of those that employs a timeout, but the supplied .I timeout argument was invalid .RI ( tv_sec was less than zero, or .IR tv_nsec was not less than 1,000,000,000). .TP .B EINVAL The operation specified in .IR futex_op employs one or both of the pointers .I uaddr and .IR uaddr2 , but one of these does not point to a valid object\(emthat is, the address is not four-byte-aligned. .TP .B EINVAL .RB ( FUTEX_WAIT_BITSET , .BR FUTEX_WAKE_BITSET ) The bit mask supplied in .IR val3 is zero. .TP .B EINVAL .RB ( FUTEX_CMP_REQUEUE_PI ) .I uaddr equals .IR uaddr2 (i.e., an attempt was made to requeue to the same futex). .TP .BR EINVAL .RB ( FUTEX_FD ) The signal number supplied in .I val is invalid. .TP .B EINVAL .RB ( FUTEX_WAKE , .BR FUTEX_WAKE_OP , .BR FUTEX_WAKE_BITSET , .BR FUTEX_REQUEUE , .BR FUTEX_CMP_REQUEUE ) The kernel detected an inconsistency between the user-space state at .I uaddr and the kernel state\(emthat is, it detected a waiter which waits in .BR FUTEX_LOCK_PI on .IR uaddr . .TP .B EINVAL .RB ( FUTEX_LOCK_PI , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_UNLOCK_PI ) The kernel detected an inconsistency between the user-space state at .I uaddr and the kernel state. This indicates either state corruption or that the kernel found a waiter on .I uaddr which is waiting via .BR FUTEX_WAIT or .BR FUTEX_WAIT_BITSET . .TP .B EINVAL .RB ( FUTEX_CMP_REQUEUE_PI ) The kernel detected an inconsistency between the user-space state at .I uaddr2 and the kernel state; .\" From a conversation with Thomas Gleixner (Aug 2015): ### .\" The kernel sees: I have non PI state for a futex you tried to .\" tell me was PI that is, the kernel detected a waiter which waits via .BR FUTEX_WAIT or .BR FUTEX_WAIT_BITSET on .IR uaddr2 . .TP .B EINVAL .RB ( FUTEX_CMP_REQUEUE_PI ) The kernel detected an inconsistency between the user-space state at .I uaddr and the kernel state; that is, the kernel detected a waiter which waits via .BR FUTEX_WAIT or .BR FUTEX_WAIT_BITESET on .IR uaddr . .TP .B EINVAL .RB ( FUTEX_CMP_REQUEUE_PI ) The kernel detected an inconsistency between the user-space state at .I uaddr and the kernel state; that is, the kernel detected a waiter which waits on .I uaddr via .BR FUTEX_LOCK_PI (instead of .BR FUTEX_WAIT_REQUEUE_PI ). .TP .B EINVAL .RB ( FUTEX_CMP_REQUEUE_PI ) .\" This deals with the case: .\" wait_requeue_pi(A, B); .\" requeue_pi(A, C); An attempt was made to requeue a waiter to a futex other than that specified by the matching .B FUTEX_WAIT_REQUEUE_PI call for that waiter. .TP .B EINVAL .RB ( FUTEX_CMP_REQUEUE_PI ) The .I val argument is not 1. .TP .B EINVAL Invalid argument. .TP .B ENFILE .RB ( FUTEX_FD ) The system-wide limit on the total number of open files has been reached. .TP .BR ENOMEM .RB ( FUTEX_LOCK_PI , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_CMP_REQUEUE_PI ) The kernel could not allocate memory to hold state information. .TP .B ENOSYS Invalid operation specified in .IR futex_op . .TP .B ENOSYS The .BR FUTEX_CLOCK_REALTIME option was specified in .IR futex_op , but the accompanying operation was neither .BR FUTEX_WAIT , .BR FUTEX_WAIT_BITSET , nor .BR FUTEX_WAIT_REQUEUE_PI . .TP .BR ENOSYS .RB ( FUTEX_LOCK_PI , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_UNLOCK_PI , .BR FUTEX_CMP_REQUEUE_PI , .BR FUTEX_WAIT_REQUEUE_PI ) A run-time check determined that the operation is not available. The PI-futex operations are not implemented on all architectures and are not supported on some CPU variants. .TP .BR EPERM .RB ( FUTEX_LOCK_PI , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_CMP_REQUEUE_PI ) The caller is not allowed to attach itself to the futex at .I uaddr (for .BR FUTEX_CMP_REQUEUE_PI : the futex at .IR uaddr2 ). (This may be caused by a state corruption in user space.) .TP .BR EPERM .RB ( FUTEX_UNLOCK_PI ) The caller does not own the lock represented by the futex word. .TP .BR ESRCH .RB ( FUTEX_LOCK_PI , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_CMP_REQUEUE_PI ) The thread ID in the futex word at .I uaddr does not exist. .TP .BR ESRCH .RB ( FUTEX_CMP_REQUEUE_PI ) The thread ID in the futex word at .I uaddr2 does not exist. .TP .B ETIMEDOUT The operation in .IR futex_op employed the timeout specified in .IR timeout , and the timeout expired before the operation completed. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .SH VERSIONS Futexes were first made available in a stable kernel release with Linux 2.6.0. .PP Initial futex support was merged in Linux 2.5.7 but with different semantics from what was described above. A four-argument system call with the semantics described in this page was introduced in Linux 2.5.40. A fifth argument was added in Linux 2.5.70, and a sixth argument was added in Linux 2.6.7. .SH CONFORMING TO This system call is Linux-specific. .SH NOTES Glibc does not provide a wrapper for this system call; call it using .BR syscall (2). .PP Several higher-level programming abstractions are implemented via futexes, including POSIX semaphores and various POSIX threads synchronization mechanisms (mutexes, condition variables, read-write locks, and barriers). .\" TODO FIXME(Torvald) Above, we cite this section and claim it contains .\" details on the synchronization semantics; add the C11 equivalents .\" here (or whatever we find consensus for). .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .SH EXAMPLES The program below demonstrates use of futexes in a program where a parent process and a child process use a pair of futexes located inside a shared anonymous mapping to synchronize access to a shared resource: the terminal. The two processes each write .IR nloops (a command-line argument that defaults to 5 if omitted) messages to the terminal and employ a synchronization protocol that ensures that they alternate in writing messages. Upon running this program we see output such as the following: .PP .in +4n .EX $ \fB./futex_demo\fP Parent (18534) 0 Child (18535) 0 Parent (18534) 1 Child (18535) 1 Parent (18534) 2 Child (18535) 2 Parent (18534) 3 Child (18535) 3 Parent (18534) 4 Child (18535) 4 .EE .in .SS Program source \& .EX /* futex_demo.c Usage: futex_demo [nloops] (Default: 5) Demonstrate the use of futexes in a program where parent and child use a pair of futexes located inside a shared anonymous mapping to synchronize access to a shared resource: the terminal. The two processes each write \(aqnum\-loops\(aq messages to the terminal and employ a synchronization protocol that ensures that they alternate in writing messages. */ #define _GNU_SOURCE #include #include #include #include #include #include #include #include #include #include #include #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \e } while (0) static uint32_t *futex1, *futex2, *iaddr; static int futex(uint32_t *uaddr, int futex_op, uint32_t val, const struct timespec *timeout, uint32_t *uaddr2, uint32_t val3) { return syscall(SYS_futex, uaddr, futex_op, val, timeout, uaddr2, val3); } /* Acquire the futex pointed to by \(aqfutexp\(aq: wait for its value to become 1, and then set the value to 0. */ static void fwait(uint32_t *futexp) { long s; /* atomic_compare_exchange_strong(ptr, oldval, newval) atomically performs the equivalent of: if (*ptr == *oldval) *ptr = newval; It returns true if the test yielded true and *ptr was updated. */ while (1) { /* Is the futex available? */ const uint32_t one = 1; if (atomic_compare_exchange_strong(futexp, &one, 0)) break; /* Yes */ /* Futex is not available; wait */ s = futex(futexp, FUTEX_WAIT, 0, NULL, NULL, 0); if (s == \-1 && errno != EAGAIN) errExit("futex\-FUTEX_WAIT"); } } /* Release the futex pointed to by \(aqfutexp\(aq: if the futex currently has the value 0, set its value to 1 and the wake any futex waiters, so that if the peer is blocked in fwait(), it can proceed. */ static void fpost(uint32_t *futexp) { long s; /* atomic_compare_exchange_strong() was described in comments above */ const uint32_t zero = 0; if (atomic_compare_exchange_strong(futexp, &zero, 1)) { s = futex(futexp, FUTEX_WAKE, 1, NULL, NULL, 0); if (s == \-1) errExit("futex\-FUTEX_WAKE"); } } int main(int argc, char *argv[]) { pid_t childPid; int nloops; setbuf(stdout, NULL); nloops = (argc > 1) ? atoi(argv[1]) : 5; /* Create a shared anonymous mapping that will hold the futexes. Since the futexes are being shared between processes, we subsequently use the "shared" futex operations (i.e., not the ones suffixed "_PRIVATE") */ iaddr = mmap(NULL, sizeof(*iaddr) * 2, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_SHARED, \-1, 0); if (iaddr == MAP_FAILED) errExit("mmap"); futex1 = &iaddr[0]; futex2 = &iaddr[1]; *futex1 = 0; /* State: unavailable */ *futex2 = 1; /* State: available */ /* Create a child process that inherits the shared anonymous mapping */ childPid = fork(); if (childPid == \-1) errExit("fork"); if (childPid == 0) { /* Child */ for (int j = 0; j < nloops; j++) { fwait(futex1); printf("Child (%jd) %d\en", (intmax_t) getpid(), j); fpost(futex2); } exit(EXIT_SUCCESS); } /* Parent falls through to here */ for (int j = 0; j < nloops; j++) { fwait(futex2); printf("Parent (%jd) %d\en", (intmax_t) getpid(), j); fpost(futex1); } wait(NULL); exit(EXIT_SUCCESS); } .EE .SH SEE ALSO .ad l .BR get_robust_list (2), .BR restart_syscall (2), .BR pthread_mutexattr_getprotocol (3), .BR futex (7), .BR sched (7) .PP The following kernel source files: .IP * 2 .I Documentation/pi-futex.txt .IP * .I Documentation/futex-requeue-pi.txt .IP * .I Documentation/locking/rt-mutex.txt .IP * .I Documentation/locking/rt-mutex-design.txt .IP * .I Documentation/robust-futex-ABI.txt .PP Franke, H., Russell, R., and Kirwood, M., 2002. \fIFuss, Futexes and Furwocks: Fast Userlevel Locking in Linux\fP (from proceedings of the Ottawa Linux Symposium 2002), .br .UR http://kernel.org\:/doc\:/ols\:/2002\:/ols2002\-pages\-479\-495.pdf .UE .PP Hart, D., 2009. \fIA futex overview and update\fP, .UR http://lwn.net/Articles/360699/ .UE .PP Hart, D.\& and Guniguntala, D., 2009. \fIRequeue-PI: Making Glibc Condvars PI-Aware\fP (from proceedings of the 2009 Real-Time Linux Workshop), .UR http://lwn.net/images/conf/rtlws11/papers/proc/p10.pdf .UE .PP Drepper, U., 2011. \fIFutexes Are Tricky\fP, .UR http://www.akkadia.org/drepper/futex.pdf .UE .PP Futex example library, futex-*.tar.bz2 at .br .UR ftp://ftp.kernel.org\:/pub\:/linux\:/kernel\:/people\:/rusty/ .UE .\" .\" FIXME(Torvald) We should probably refer to the glibc code here, in .\" particular the glibc-internal futex wrapper functions that are .\" WIP, and the generic pthread_mutex_t and perhaps condvar .\" implementations. .SH COLOPHON This page is part of release 5.10 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.