.\" Copyright (c) 2007 Stephan Uphoff .\" Copyright (c) 2006 Gleb Smirnoff .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" $FreeBSD: src/share/man/man9/rmlock.9,v 1.5.2.1.6.1 2010/12/21 17:09:25 kensmith Exp $ .\" .\" Based on rwlock.9 man page .Dd November 10, 2007 .Dt RMLOCK 9 .Os .Sh NAME .Nm rmlock , .Nm rm_init , .Nm rm_init_flags , .Nm rm_destroy , .Nm rm_rlock , .Nm rm_wlock , .Nm rm_runlock , .Nm rm_wunlock , .Nm rm_wowned , .Nm RM_SYSINIT .Nd kernel reader/writer lock optimized for mostly read access patterns .Sh SYNOPSIS .In sys/param.h .In sys/lock.h .In sys/rmlock.h .Ft void .Fn rm_init "struct rmlock *rm" "const char *name" .Ft void .Fn rm_init_flags "struct rmlock *rm" "const char *name" "int opts" .Ft void .Fn rm_destroy "struct rmlock *rm" .Ft void .Fn rm_rlock "struct rmlock *rm" "struct rm_priotracker* tracker" .Ft void .Fn rm_wlock "struct rmlock *rm" .Ft void .Fn rm_runlock "struct rmlock *rm" "struct rm_priotracker* tracker" .Ft void .Fn rm_wunlock "struct rmlock *rm" .Ft int .Fn rm_wowned "struct rmlock *rm" .In sys/kernel.h .Fn RM_SYSINIT "name" "struct rmlock *rm" "const char *desc" "int opts" .Sh DESCRIPTION Mostly reader 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 .Em readers since they only read the protected data. A thread with exclusive access is known as a .Em writer since it can modify protected data. .Pp 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 had been locked for shared access is an expensive operation. .Pp Although reader/writer locks look very similar to .Xr sx 9 locks, their usage pattern is different. Reader/writer locks can be treated as mutexes (see .Xr mutex 9 ) with shared/exclusive semantics. Unlike .Xr sx 9 , an .Nm can be locked while holding a non-spin mutex, and an .Nm cannot be held while sleeping. The .Nm locks have full priority propagation like mutexes. The .Va rm_priotracker structure argument supplied in .Fn rm_rlock and .Fn rm_runlock is used to keep track of the read owner(s). Another important property is that shared holders of .Nm can recurse if the lock has been initialized with the .Dv LO_RECURSABLE option, however exclusive locks are not allowed to recurse. .Ss Macros and Functions .Bl -tag -width indent .It Fn rm_init "struct rmlock *rm" "const char *name" Initialize structure located at .Fa rm as mostly reader lock, described by .Fa name . The name description is used solely for debugging purposes. This function must be called before any other operations on the lock. .It Fn rm_init_flags "struct rmlock *rm" "const char *name" "int opts" Initialize the rm lock just like the .Fn rm_init function, but specifying a set of optional flags to alter the behaviour of .Fa rm , through the .Fa opts argument. It contains one or more of the following flags: .Bl -tag -width ".Dv RM_NOWITNESS" .It Dv RM_NOWITNESS Instruct .Xr witness 4 to ignore this lock. .It Dv RM_RECURSE Allow threads to recursively acquire exclusive locks for .Fa rm . .El .It Fn rm_rlock "struct rmlock *rm" "struct rm_priotracker* tracker" Lock .Fa rm as a reader. Using .Fa tracker to track read owners of a lock for priority propagation. This data structure is only used internally by .Nm and must persist until .Fn rm_runlock has been called. This data structure can be allocated on the stack since rmlocks cannot be held while sleeping. 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 .Dv LO_RECURSABLE option the .Fn rm_rlock function can be called when the thread has already acquired reader access on .Fa rm . This is called .Dq "recursing on a lock" . .It Fn rm_wlock "struct rmlock *rm" Lock .Fa rm as a writer. If there are any shared owners of the lock, the current thread blocks. The .Fn rm_wlock function cannot be called recursively. .It Fn rm_runlock "struct rmlock *rm" "struct rm_priotracker* tracker" This function releases a shared lock previously acquired by .Fn rm_rlock . The .Fa tracker argument must match the .Fa tracker argument used for acquiring the shared lock .It Fn rm_wunlock "struct rmlock *rm" This function releases an exclusive lock previously acquired by .Fn rm_wlock . .It Fn rm_destroy "struct rmlock *rm" This functions destroys a lock previously initialized with .Fn rm_init . The .Fa rm lock must be unlocked. .It Fn rm_wowned "struct rmlock *rm" This function returns a non-zero value if the current thread owns an exclusive lock on .Fa rm . .El .Sh SEE ALSO .Xr locking 9 , .Xr mutex 9 , .Xr panic 9 , .Xr rwlock 9 , .Xr sema 9 , .Xr sx 9 .Sh HISTORY These functions appeared in .Fx 7.0 . .Sh AUTHORS .An -nosplit The .Nm facility was written by .An "Stephan Uphoff" . This manual page was written by .An "Gleb Smirnoff" for rwlock and modified to reflect rmlock by .An "Stephan Uphoff" . .Sh BUGS The .Nm implementation is currently not optimized for single processor systems. .Pp The .Nm implementation 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. .Pp The .Nm can currently not be used as a lock argument for condition variable wait functions.