1.\" Copyright (c) 2007 Stephan Uphoff <ups@FreeBSD.org> 2.\" Copyright (c) 2006 Gleb Smirnoff <glebius@FreeBSD.org> 3.\" All rights reserved. 4.\" 5.\" Redistribution and use in source and binary forms, with or without 6.\" modification, are permitted provided that the following conditions 7.\" are met: 8.\" 1. Redistributions of source code must retain the above copyright 9.\" notice, this list of conditions and the following disclaimer. 10.\" 2. Redistributions in binary form must reproduce the above copyright 11.\" notice, this list of conditions and the following disclaimer in the 12.\" documentation and/or other materials provided with the distribution. 13.\" 14.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 15.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 16.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 17.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 18.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 19.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 20.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 21.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 22.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 23.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 24.\" SUCH DAMAGE. 25.\" 26.\" $FreeBSD$ 27.\" 28.\" Based on rwlock.9 man page 29.Dd November 10, 2007 30.Dt RMLOCK 9 31.Os 32.Sh NAME 33.Nm rmlock , 34.Nm rm_init , 35.Nm rm_init_flags , 36.Nm rm_destroy , 37.Nm rm_rlock , 38.Nm rm_wlock , 39.Nm rm_runlock , 40.Nm rm_wunlock , 41.Nm rm_wowned , 42.Nm RM_SYSINIT 43.Nd kernel reader/writer lock optimized for mostly read access patterns 44.Sh SYNOPSIS 45.In sys/param.h 46.In sys/lock.h 47.In sys/rmlock.h 48.Ft void 49.Fn rm_init "struct rmlock *rm" "const char *name" 50.Ft void 51.Fn rm_init_flags "struct rmlock *rm" "const char *name" "int opts" 52.Ft void 53.Fn rm_destroy "struct rmlock *rm" 54.Ft void 55.Fn rm_rlock "struct rmlock *rm" "struct rm_priotracker* tracker" 56.Ft void 57.Fn rm_wlock "struct rmlock *rm" 58.Ft void 59.Fn rm_runlock "struct rmlock *rm" "struct rm_priotracker* tracker" 60.Ft void 61.Fn rm_wunlock "struct rmlock *rm" 62.Ft int 63.Fn rm_wowned "struct rmlock *rm" 64.In sys/kernel.h 65.Fn RM_SYSINIT "name" "struct rmlock *rm" "const char *desc" "int opts" 66.Sh DESCRIPTION 67Mostly reader locks allow shared access to protected data by multiple threads, 68or exclusive access by a single thread. 69The threads with shared access are known as 70.Em readers 71since they only read the protected data. 72A thread with exclusive access is known as a 73.Em writer 74since it can modify protected data. 75.Pp 76Read mostly locks are designed to be efficient for locks almost exclusively 77used as reader locks and as such should be used for protecting data that 78rarely changes. 79Acquiring an exclusive lock after the lock had been locked for shared access 80is an expensive operation. 81.Pp 82Although reader/writer locks look very similar to 83.Xr sx 9 84locks, their usage pattern is different. 85Reader/writer locks can be treated as mutexes (see 86.Xr mutex 9 ) 87with shared/exclusive semantics. 88Unlike 89.Xr sx 9 , 90an 91.Nm 92can be locked while holding a non-spin mutex, and an 93.Nm 94cannot be held while sleeping. 95The 96.Nm 97locks have full priority propagation like mutexes. 98The 99.Va rm_priotracker 100structure argument supplied in 101.Fn rm_rlock 102and 103.Fn rm_runlock 104is used to keep track of the read owner(s). 105Another important property is that shared holders of 106.Nm 107can recurse if the lock has been initialized with the 108.Dv LO_RECURSABLE 109option, however exclusive locks are not allowed to recurse. 110.Ss Macros and Functions 111.Bl -tag -width indent 112.It Fn rm_init "struct rmlock *rm" "const char *name" 113Initialize structure located at 114.Fa rm 115as mostly reader lock, described by 116.Fa name . 117The name description is used solely for debugging purposes. 118This function must be called before any other operations 119on the lock. 120.It Fn rm_init_flags "struct rmlock *rm" "const char *name" "int opts" 121Initialize the rm lock just like the 122.Fn rm_init 123function, but specifying a set of optional flags to alter the 124behaviour of 125.Fa rm , 126through the 127.Fa opts 128argument. 129It contains one or more of the following flags: 130.Bl -tag -width ".Dv RM_NOWITNESS" 131.It Dv RM_NOWITNESS 132Instruct 133.Xr witness 4 134to ignore this lock. 135.It Dv RM_RECURSE 136Allow threads to recursively acquire exclusive locks for 137.Fa rm . 138.El 139.It Fn rm_rlock "struct rmlock *rm" "struct rm_priotracker* tracker" 140Lock 141.Fa rm 142as a reader. 143Using 144.Fa tracker 145to track read owners of a lock for priority propagation. 146This data structure is only used internally by 147.Nm 148and must persist until 149.Fn rm_runlock 150has been called. 151This data structure can be allocated on the stack since 152rmlocks cannot be held while sleeping. 153If any thread holds this lock exclusively, the current thread blocks, 154and its priority is propagated to the exclusive holder. 155If the lock was initialized with the 156.Dv LO_RECURSABLE 157option the 158.Fn rm_rlock 159function can be called when the thread has already acquired reader 160access on 161.Fa rm . 162This is called 163.Dq "recursing on a lock" . 164.It Fn rm_wlock "struct rmlock *rm" 165Lock 166.Fa rm 167as a writer. 168If there are any shared owners of the lock, the current thread blocks. 169The 170.Fn rm_wlock 171function cannot be called recursively. 172.It Fn rm_runlock "struct rmlock *rm" "struct rm_priotracker* tracker" 173This function releases a shared lock previously acquired by 174.Fn rm_rlock . 175The 176.Fa tracker 177argument must match the 178.Fa tracker 179argument used for acquiring the shared lock 180.It Fn rm_wunlock "struct rmlock *rm" 181This function releases an exclusive lock previously acquired by 182.Fn rm_wlock . 183.It Fn rm_destroy "struct rmlock *rm" 184This functions destroys a lock previously initialized with 185.Fn rm_init . 186The 187.Fa rm 188lock must be unlocked. 189.It Fn rm_wowned "struct rmlock *rm" 190This function returns a non-zero value if the current thread owns an 191exclusive lock on 192.Fa rm . 193.El 194.Sh SEE ALSO 195.Xr locking 9 , 196.Xr mutex 9 , 197.Xr panic 9 , 198.Xr rwlock 9 , 199.Xr sema 9 , 200.Xr sx 9 201.Sh HISTORY 202These 203functions appeared in 204.Fx 7.0 . 205.Sh AUTHORS 206.An -nosplit 207The 208.Nm 209facility was written by 210.An "Stephan Uphoff" . 211This manual page was written by 212.An "Gleb Smirnoff" 213for rwlock and modified to reflect rmlock by 214.An "Stephan Uphoff" . 215.Sh BUGS 216The 217.Nm 218implementation is currently not optimized for single processor systems. 219.Pp 220The 221.Nm 222implementation uses a single per CPU list shared by all 223rmlocks in the system. 224If rmlocks become popular, hashing to multiple per CPU queues may 225be needed to speed up the writer lock process. 226.Pp 227The 228.Nm 229can currently not be used as a lock argument for condition variable 230wait functions. 231