|RMLOCK(9)||Kernel Developer's Manual||RMLOCK(9)|
kernel reader/writer lock optimized for read-mostly access patterns
rm_init(struct rmlock *rm, const char *name); void
rm_init_flags(struct rmlock *rm, const char *name, int opts); void
rm_destroy(struct rmlock *rm); void
rm_rlock(struct rmlock *rm, struct rm_priotracker* tracker); int
rm_try_rlock(struct rmlock *rm, struct rm_priotracker* tracker); void
rm_wlock(struct rmlock *rm); void
rm_runlock(struct rmlock *rm, struct rm_priotracker* tracker); void
rm_wunlock(struct rmlock *rm); int
rm_wowned(const struct rmlock *rm); int
rm_sleep(void *wchan, struct rmlock *rm, int priority, const char *wmesg, int timo);
rm_assert(struct rmlock *rm, int what);
RM_SYSINIT(name, struct rmlock *rm, const char *desc, int opts);
DESCRIPTION¶Read-mostly locks allow shared access to protected data by multiple threads, or exclusive access by a single thread. The threads with shared access are known as readers since they only read the protected data. A thread with exclusive access is known as a writer since it can modify protected data. Read-mostly locks are designed to be efficient for locks almost exclusively used as reader locks and as such should be used for protecting data that rarely changes. Acquiring an exclusive lock after the lock has been locked for shared access is an expensive operation. Normal read-mostly locks are similar to rwlock(9) locks and follow the same lock ordering rules as rwlock(9) locks. Read-mostly locks have full priority propagation like mutexes. Unlike rwlock(9), read-mostly locks propagate priority to both readers and writers. This is implemented via the rm_priotracker structure argument supplied to
rm_runlock(). Readers can recurse if the lock is initialized with the
RM_RECURSEoption; however, writers are never allowed to recurse. Sleepable read-mostly locks are created by passing
rm_init_flags(). Unlike normal read-mostly locks, sleepable read-mostly locks follow the same lock ordering rules as sx(9) locks. Sleepable read-mostly locks do not propagate priority to writers, but they do propagate priority to readers. Writers are permitted to sleep while holding a read-mostly lock, but readers are not. Unlike other sleepable locks such as sx(9) locks, readers must use try operations on other sleepable locks to avoid sleeping.
Macros and Functions¶
rm_init(struct rmlock *rm, const char *name)
- Initialize the read-mostly lock rm. The name description is used solely for debugging purposes. This function must be called before any other operations on the lock.
rm_init_flags(struct rmlock *rm, const char *name, int opts)
- Similar to
rm_init(), initialize the read-mostly lock rm with a set of optional flags. The opts arguments contains one or more of the following flags:
- Instruct witness(4) to ignore this lock.
- Allow threads to recursively acquire shared locks for rm.
- Create a sleepable read-mostly lock.
rm_rlock(struct rmlock *rm, struct rm_priotracker* tracker)
- Lock rm as a reader using
tracker to track read owners of a lock
for priority propagation. This data structure is only used internally by
rmlockand must persist until
rm_runlock() has been called. This data structure can be allocated on the stack since readers cannot sleep. If any thread holds this lock exclusively, the current thread blocks, and its priority is propagated to the exclusive holder. If the lock was initialized with the
rm_rlock() function can be called when the current thread has already acquired reader access on rm.
rm_try_rlock(struct rmlock *rm, struct rm_priotracker* tracker)
- Try to lock rm as a reader.
rm_try_rlock() will return 0 if the lock cannot be acquired immediately; otherwise, the lock will be acquired and a non-zero value will be returned. Note that
rm_try_rlock() may fail even while the lock is not currently held by a writer. If the lock was initialized with the
rm_try_rlock() will succeed if the current thread has already acquired reader access.
rm_wlock(struct rmlock *rm)
- Lock rm as a writer. If there are any
shared owners of the lock, the current thread blocks. The
rm_wlock() function cannot be called recursively.
rm_runlock(struct rmlock *rm, struct rm_priotracker* tracker)
- This function releases a shared lock previously acquired by
rm_rlock(). The tracker argument must match the tracker argument used for acquiring the shared lock
rm_wunlock(struct rmlock *rm)
- This function releases an exclusive lock previously acquired by
rm_destroy(struct rmlock *rm)
- This functions destroys a lock previously initialized with
rm_init(). The rm lock must be unlocked.
rm_wowned(const struct rmlock *rm)
- This function returns a non-zero value if the current thread owns an exclusive lock on rm.
rm_sleep(void *wchan, struct rmlock *rm, int priority, const char *wmesg, int timo)
- This function atomically releases rm while waiting for an event. The rm lock must be exclusively locked. For more details on the parameters to this function, see sleep(9).
rm_assert(struct rmlock *rm, int what)
- This function asserts that the rm lock is
in the state specified by what. If the
assertions are not true and the kernel is compiled with
options INVARIANT_SUPPORT, the kernel will panic. Currently the following base assertions are supported:
- Assert that current thread holds either a shared or exclusive lock of rm.
- Assert that current thread holds a shared lock of rm.
- Assert that current thread holds an exclusive lock of rm.
- Assert that current thread holds neither a shared nor exclusive lock of rm.
- Assert that the current thread holds a recursive lock of rm.
- Assert that the current thread does not hold a recursive lock of rm.
SEE ALSO¶locking(9), mutex(9), panic(9), rwlock(9), sleep(9), sema(9), sx(9)
HISTORY¶These functions appeared in FreeBSD 7.0.
rmlockfacility was written by Stephan Uphoff. This manual page was written by Gleb Smirnoff for rwlock and modified to reflect rmlock by Stephan Uphoff.
rmlockimplementation is currently not optimized for single processor systems.
rm_try_rlock() can fail transiently even when there is no writer, while another reader updates the state on the local CPU. The
rmlockimplementation uses a single per CPU list shared by all rmlocks in the system. If rmlocks become popular, hashing to multiple per CPU queues may be needed to speed up the writer lock process.
|June 25, 2013||Debian|