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 December 27, 2019 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_try_rlock , 39.Nm rm_wlock , 40.Nm rm_runlock , 41.Nm rm_wunlock , 42.Nm rm_wowned , 43.Nm rm_sleep , 44.Nm rm_assert , 45.Nm RM_SYSINIT , 46.Nm RM_SYSINIT_FLAGS , 47.Nm rms_init , 48.Nm rms_destroy , 49.Nm rms_rlock , 50.Nm rms_wlock , 51.Nm rms_runlock , 52.Nm rms_wunlock 53.Nd kernel reader/writer lock optimized for read-mostly access patterns 54.Sh SYNOPSIS 55.In sys/param.h 56.In sys/lock.h 57.In sys/rmlock.h 58.Ft void 59.Fn rm_init "struct rmlock *rm" "const char *name" 60.Ft void 61.Fn rm_init_flags "struct rmlock *rm" "const char *name" "int opts" 62.Ft void 63.Fn rm_destroy "struct rmlock *rm" 64.Ft void 65.Fn rm_rlock "struct rmlock *rm" "struct rm_priotracker* tracker" 66.Ft int 67.Fn rm_try_rlock "struct rmlock *rm" "struct rm_priotracker* tracker" 68.Ft void 69.Fn rm_wlock "struct rmlock *rm" 70.Ft void 71.Fn rm_runlock "struct rmlock *rm" "struct rm_priotracker* tracker" 72.Ft void 73.Fn rm_wunlock "struct rmlock *rm" 74.Ft int 75.Fn rm_wowned "const struct rmlock *rm" 76.Ft int 77.Fn rm_sleep "void *wchan" "struct rmlock *rm" "int priority" "const char *wmesg" "int timo" 78.Pp 79.Cd "options INVARIANTS" 80.Cd "options INVARIANT_SUPPORT" 81.Ft void 82.Fn rm_assert "struct rmlock *rm" "int what" 83.In sys/kernel.h 84.Fn RM_SYSINIT "name" "struct rmlock *rm" "const char *desc" 85.Fn RM_SYSINIT_FLAGS "name" "struct rmlock *rm" "const char *desc" "int flags" 86.Ft void 87.Fn rms_init "struct rmslock *rms" "const char *name" 88.Ft void 89.Fn rms_destroy "struct rmslock *rms" 90.Ft void 91.Fn rms_rlock "struct rmslock *rms" 92.Ft void 93.Fn rms_wlock "struct rmslock *rms" 94.Ft void 95.Fn rms_runlock "struct rmslock *rms" 96.Ft void 97.Fn rms_wunlock "struct rmslock *rms" 98.Sh DESCRIPTION 99Read-mostly locks allow shared access to protected data by multiple threads, 100or exclusive access by a single thread. 101The threads with shared access are known as 102.Em readers 103since they only read the protected data. 104A thread with exclusive access is known as a 105.Em writer 106since it can modify protected data. 107.Pp 108Read-mostly locks are designed to be efficient for locks almost exclusively 109used as reader locks and as such should be used for protecting data that 110rarely changes. 111Acquiring an exclusive lock after the lock has been locked for shared access 112is an expensive operation. 113.Pp 114Normal read-mostly locks are similar to 115.Xr rwlock 9 116locks and follow the same lock ordering rules as 117.Xr rwlock 9 118locks. 119Read-mostly locks have full priority propagation like mutexes. 120Unlike 121.Xr rwlock 9 , 122read-mostly locks propagate priority to both readers and writers. 123This is implemented via the 124.Va rm_priotracker 125structure argument supplied to 126.Fn rm_rlock 127and 128.Fn rm_runlock . 129Readers can recurse if the lock is initialized with the 130.Dv RM_RECURSE 131option; 132however, writers are never allowed to recurse. 133.Pp 134Sleeping for writers can be allowed by passing 135.Dv RM_SLEEPABLE 136to 137.Fn rm_init_flags . 138It changes lock ordering rules to the same as for 139.Xr sx 9 140locks. 141They do not propagate priority to writers, but they do propagate priority to readers. 142Note that readers are not permitted to sleep regardless of the flag. 143.Pp 144Sleepable read-mostly locks (created with 145.Fn rms_init ) 146allow sleeping for both readers and writers, but don't do priority propagation 147for either. 148They follow 149.Xr sx 9 150lock ordering. 151.Ss Macros and Functions 152.Bl -tag -width indent 153.It Fn rm_init "struct rmlock *rm" "const char *name" 154Initialize the read-mostly lock 155.Fa rm . 156The 157.Fa name 158description is used solely for debugging purposes. 159This function must be called before any other operations 160on the lock. 161.It Fn rm_init_flags "struct rmlock *rm" "const char *name" "int opts" 162Similar to 163.Fn rm_init , 164initialize the read-mostly lock 165.Fa rm 166with a set of optional flags. 167The 168.Fa opts 169arguments contains one or more of the following flags: 170.Bl -tag -width ".Dv RM_NOWITNESS" 171.It Dv RM_NOWITNESS 172Instruct 173.Xr witness 4 174to ignore this lock. 175.It Dv RM_RECURSE 176Allow threads to recursively acquire shared locks for 177.Fa rm . 178.It Dv RM_SLEEPABLE 179Create a sleepable read-mostly lock. 180.It Dv RM_NEW 181If the kernel has been compiled with 182.Cd "option INVARIANTS" , 183.Fn rm_init_flags 184will assert that the 185.Fa rm 186has not been initialized multiple times without intervening calls to 187.Fn rm_destroy 188unless this option is specified. 189.El 190.It Fn rm_rlock "struct rmlock *rm" "struct rm_priotracker* tracker" 191Lock 192.Fa rm 193as a reader using 194.Fa tracker 195to track read owners of a lock for priority propagation. 196This data structure is only used internally by 197.Nm 198and must persist until 199.Fn rm_runlock 200has been called. 201This data structure can be allocated on the stack since 202readers cannot sleep. 203If any thread holds this lock exclusively, the current thread blocks, 204and its priority is propagated to the exclusive holder. 205If the lock was initialized with the 206.Dv RM_RECURSE 207option the 208.Fn rm_rlock 209function can be called when the current thread has already acquired reader 210access on 211.Fa rm . 212.It Fn rm_try_rlock "struct rmlock *rm" "struct rm_priotracker* tracker" 213Try to lock 214.Fa rm 215as a reader. 216.Fn rm_try_rlock 217will return 0 if the lock cannot be acquired immediately; 218otherwise, 219the lock will be acquired and a non-zero value will be returned. 220Note that 221.Fn rm_try_rlock 222may fail even while the lock is not currently held by a writer. 223If the lock was initialized with the 224.Dv RM_RECURSE 225option, 226.Fn rm_try_rlock 227will succeed if the current thread has already acquired reader access. 228.It Fn rm_wlock "struct rmlock *rm" 229Lock 230.Fa rm 231as a writer. 232If there are any shared owners of the lock, the current thread blocks. 233The 234.Fn rm_wlock 235function cannot be called recursively. 236.It Fn rm_runlock "struct rmlock *rm" "struct rm_priotracker* tracker" 237This function releases a shared lock previously acquired by 238.Fn rm_rlock . 239The 240.Fa tracker 241argument must match the 242.Fa tracker 243argument used for acquiring the shared lock 244.It Fn rm_wunlock "struct rmlock *rm" 245This function releases an exclusive lock previously acquired by 246.Fn rm_wlock . 247.It Fn rm_destroy "struct rmlock *rm" 248This functions destroys a lock previously initialized with 249.Fn rm_init . 250The 251.Fa rm 252lock must be unlocked. 253.It Fn rm_wowned "const struct rmlock *rm" 254This function returns a non-zero value if the current thread owns an 255exclusive lock on 256.Fa rm . 257.It Fn rm_sleep "void *wchan" "struct rmlock *rm" "int priority" "const char *wmesg" "int timo" 258This function atomically releases 259.Fa rm 260while waiting for an event. 261The 262.Fa rm 263lock must be exclusively locked. 264For more details on the parameters to this function, 265see 266.Xr sleep 9 . 267.It Fn rm_assert "struct rmlock *rm" "int what" 268This function asserts that the 269.Fa rm 270lock is in the state specified by 271.Fa what . 272If the assertions are not true and the kernel is compiled with 273.Cd "options INVARIANTS" 274and 275.Cd "options INVARIANT_SUPPORT" , 276the kernel will panic. 277Currently the following base assertions are supported: 278.Bl -tag -width ".Dv RA_UNLOCKED" 279.It Dv RA_LOCKED 280Assert that current thread holds either a shared or exclusive lock 281of 282.Fa rm . 283.It Dv RA_RLOCKED 284Assert that current thread holds a shared lock of 285.Fa rm . 286.It Dv RA_WLOCKED 287Assert that current thread holds an exclusive lock of 288.Fa rm . 289.It Dv RA_UNLOCKED 290Assert that current thread holds neither a shared nor exclusive lock of 291.Fa rm . 292.El 293.Pp 294In addition, one of the following optional flags may be specified with 295.Dv RA_LOCKED , 296.Dv RA_RLOCKED , 297or 298.Dv RA_WLOCKED : 299.Bl -tag -width ".Dv RA_NOTRECURSED" 300.It Dv RA_RECURSED 301Assert that the current thread holds a recursive lock of 302.Fa rm . 303.It Dv RA_NOTRECURSED 304Assert that the current thread does not hold a recursive lock of 305.Fa rm . 306.El 307.El 308.Bl -tag -width indent 309.It Fn rms_init "struct rmslock *rms" "const char *name" 310Initialize the sleepable read-mostly lock 311.Fa rms . 312The 313.Fa name 314description is used as 315.Fa wmesg 316parameter to the 317.Xr msleep 9 318routine. 319This function must be called before any other operations on the lock. 320.It Fn rms_rlock "struct rmlock *rm" 321Lock 322.Fa rms 323as a reader. 324If any thread holds this lock exclusively, the current thread blocks. 325.It Fn rms_wlock "struct rmslock *rms" 326Lock 327.Fa rms 328as a writer. 329If the lock is already taken, the current thread blocks. 330The 331.Fn rms_wlock 332function cannot be called recursively. 333.It Fn rms_runlock "struct rmslock *rms" 334This function releases a shared lock previously acquired by 335.Fn rms_rlock . 336.It Fn rms_wunlock "struct rmslock *rms" 337This function releases an exclusive lock previously acquired by 338.Fn rms_wlock . 339.It Fn rms_destroy "struct rmslock *rms" 340This functions destroys a lock previously initialized with 341.Fn rms_init . 342The 343.Fa rms 344lock must be unlocked. 345.Sh SEE ALSO 346.Xr locking 9 , 347.Xr mutex 9 , 348.Xr panic 9 , 349.Xr rwlock 9 , 350.Xr sema 9 , 351.Xr sleep 9 , 352.Xr sx 9 353.Sh HISTORY 354These functions appeared in 355.Fx 7.0 . 356.Sh AUTHORS 357.An -nosplit 358The 359.Nm 360facility was written by 361.An "Stephan Uphoff" . 362This manual page was written by 363.An "Gleb Smirnoff" 364for rwlock and modified to reflect rmlock by 365.An "Stephan Uphoff" . 366.Sh BUGS 367The 368.Nm 369implementation is currently not optimized for single processor systems. 370.Pp 371.Fn rm_try_rlock 372can fail transiently even when there is no writer, while another reader 373updates the state on the local CPU. 374.Pp 375The 376.Nm 377implementation uses a single per CPU list shared by all 378rmlocks in the system. 379If rmlocks become popular, hashing to multiple per CPU queues may 380be needed to speed up the writer lock process. 381