1.\" Copyright (c) 2006 Gleb Smirnoff <glebius@FreeBSD.org> 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 17.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\" SUCH DAMAGE. 24.\" 25.Dd November 11, 2017 26.Dt RWLOCK 9 27.Os 28.Sh NAME 29.Nm rwlock , 30.Nm rw_init , 31.Nm rw_init_flags , 32.Nm rw_destroy , 33.Nm rw_rlock , 34.Nm rw_wlock , 35.Nm rw_runlock , 36.Nm rw_wunlock , 37.Nm rw_unlock , 38.Nm rw_try_rlock , 39.Nm rw_try_upgrade , 40.Nm rw_try_wlock , 41.Nm rw_downgrade , 42.Nm rw_sleep , 43.Nm rw_initialized , 44.Nm rw_wowned , 45.Nm rw_assert , 46.Nm RW_SYSINIT , 47.Nm RW_SYSINIT_FLAGS 48.Nd kernel reader/writer lock 49.Sh SYNOPSIS 50.In sys/param.h 51.In sys/lock.h 52.In sys/rwlock.h 53.Ft void 54.Fn rw_init "struct rwlock *rw" "const char *name" 55.Ft void 56.Fn rw_init_flags "struct rwlock *rw" "const char *name" "int opts" 57.Ft void 58.Fn rw_destroy "struct rwlock *rw" 59.Ft void 60.Fn rw_rlock "struct rwlock *rw" 61.Ft void 62.Fn rw_wlock "struct rwlock *rw" 63.Ft int 64.Fn rw_try_rlock "struct rwlock *rw" 65.Ft int 66.Fn rw_try_wlock "struct rwlock *rw" 67.Ft void 68.Fn rw_runlock "struct rwlock *rw" 69.Ft void 70.Fn rw_wunlock "struct rwlock *rw" 71.Ft void 72.Fn rw_unlock "struct rwlock *rw" 73.Ft int 74.Fn rw_try_upgrade "struct rwlock *rw" 75.Ft void 76.Fn rw_downgrade "struct rwlock *rw" 77.Ft int 78.Fn rw_sleep "void *chan" "struct rwlock *rw" "int priority" "const char *wmesg" "int timo" 79.Ft int 80.Fn rw_initialized "const struct rwlock *rw" 81.Ft int 82.Fn rw_wowned "const struct rwlock *rw" 83.Pp 84.Cd "options INVARIANTS" 85.Cd "options INVARIANT_SUPPORT" 86.Ft void 87.Fn rw_assert "const struct rwlock *rw" "int what" 88.In sys/kernel.h 89.Fn RW_SYSINIT "name" "struct rwlock *rw" "const char *desc" 90.Fn RW_SYSINIT_FLAGS "name" "struct rwlock *rw" "const char *desc" "int flags" 91.Sh DESCRIPTION 92Reader/writer locks allow shared access to protected data by multiple threads, 93or exclusive access by a single thread. 94The threads with shared access are known as 95.Em readers 96since they only read the protected data. 97A thread with exclusive access is known as a 98.Em writer 99since it can modify protected data. 100.Pp 101Although reader/writer locks look very similar to 102.Xr sx 9 103locks, their usage pattern is different. 104Reader/writer locks can be treated as mutexes (see 105.Xr mutex 9 ) 106with shared/exclusive semantics. 107Unlike 108.Xr sx 9 , 109an 110.Nm 111can be locked while holding a non-spin mutex, and an 112.Nm 113cannot be held while sleeping. 114The 115.Nm 116locks have priority propagation like mutexes, but priority 117can be propagated only to writers. 118This limitation comes from the fact that readers 119are anonymous. 120Another important property is that readers can always recurse, 121and exclusive locks can be made recursive selectively. 122.Ss Macros and Functions 123.Bl -tag -width indent 124.It Fn rw_init "struct rwlock *rw" "const char *name" 125Initialize structure located at 126.Fa rw 127as reader/writer lock, described by name 128.Fa name . 129The description is used solely for debugging purposes. 130This function must be called before any other operations 131on the lock. 132.It Fn rw_init_flags "struct rwlock *rw" "const char *name" "int opts" 133Initialize the rw lock just like the 134.Fn rw_init 135function, but specifying a set of optional flags to alter the 136behaviour of 137.Fa rw , 138through the 139.Fa opts 140argument. 141It contains one or more of the following flags: 142.Bl -tag -width ".Dv RW_NOPROFILE" 143.It Dv RW_DUPOK 144Witness should not log messages about duplicate locks being acquired. 145.It Dv RW_NOPROFILE 146Do not profile this lock. 147.It Dv RW_NOWITNESS 148Instruct 149.Xr witness 4 150to ignore this lock. 151.It Dv RW_QUIET 152Do not log any operations for this lock via 153.Xr ktr 4 . 154.It Dv RW_RECURSE 155Allow threads to recursively acquire exclusive locks for 156.Fa rw . 157.It Dv RW_NEW 158If the kernel has been compiled with 159.Cd "option INVARIANTS" , 160.Fn rw_init_flags 161will assert that the 162.Fa rw 163has not been initialized multiple times without intervening calls to 164.Fn rw_destroy 165unless this option is specified. 166.El 167.It Fn rw_rlock "struct rwlock *rw" 168Lock 169.Fa rw 170as a reader. 171If any thread holds this lock exclusively, the current thread blocks, 172and its priority is propagated to the exclusive holder. 173The 174.Fn rw_rlock 175function can be called when the thread has already acquired reader 176access on 177.Fa rw . 178This is called 179.Dq "recursing on a lock" . 180.It Fn rw_wlock "struct rwlock *rw" 181Lock 182.Fa rw 183as a writer. 184If there are any shared owners of the lock, the current thread blocks. 185The 186.Fn rw_wlock 187function can be called recursively only if 188.Fa rw 189has been initialized with the 190.Dv RW_RECURSE 191option enabled. 192.It Fn rw_try_rlock "struct rwlock *rw" 193Try to lock 194.Fa rw 195as a reader. 196This function will return true if the operation succeeds, otherwise 0 197will be returned. 198.It Fn rw_try_wlock "struct rwlock *rw" 199Try to lock 200.Fa rw 201as a writer. 202This function will return true if the operation succeeds, otherwise 0 203will be returned. 204.It Fn rw_runlock "struct rwlock *rw" 205This function releases a shared lock previously acquired by 206.Fn rw_rlock . 207.It Fn rw_wunlock "struct rwlock *rw" 208This function releases an exclusive lock previously acquired by 209.Fn rw_wlock . 210.It Fn rw_unlock "struct rwlock *rw" 211This function releases a shared lock previously acquired by 212.Fn rw_rlock 213or an exclusive lock previously acquired by 214.Fn rw_wlock . 215.It Fn rw_try_upgrade "struct rwlock *rw" 216Attempt to upgrade a single shared lock to an exclusive lock. 217The current thread must hold a shared lock of 218.Fa rw . 219This will only succeed if the current thread holds the only shared lock on 220.Fa rw , 221and it only holds a single shared lock. 222If the attempt succeeds 223.Fn rw_try_upgrade 224will return a non-zero value, 225and the current thread will hold an exclusive lock. 226If the attempt fails 227.Fn rw_try_upgrade 228will return zero, 229and the current thread will still hold a shared lock. 230.It Fn rw_downgrade "struct rwlock *rw" 231Convert an exclusive lock into a single shared lock. 232The current thread must hold an exclusive lock of 233.Fa rw . 234.It Fn rw_sleep "void *chan" "struct rwlock *rw" "int priority" "const char *wmesg" "int timo" 235Atomically release 236.Fa rw 237while waiting for an event. 238For more details on the parameters to this function, 239see 240.Xr sleep 9 . 241.It Fn rw_initialized "const struct rwlock *rw" 242This function returns non-zero if 243.Fa rw 244has been initialized, and zero otherwise. 245.It Fn rw_destroy "struct rwlock *rw" 246This functions destroys a lock previously initialized with 247.Fn rw_init . 248The 249.Fa rw 250lock must be unlocked. 251.It Fn rw_wowned "const struct rwlock *rw" 252This function returns a non-zero value if the current thread owns an 253exclusive lock on 254.Fa rw . 255.It Fn rw_assert "const struct rwlock *rw" "int what" 256This function allows assertions specified in 257.Fa what 258to be made about 259.Fa rw . 260If the assertions are not true and the kernel is compiled 261with 262.Cd "options INVARIANTS" 263and 264.Cd "options INVARIANT_SUPPORT" , 265the kernel will panic. 266Currently the following base assertions are supported: 267.Bl -tag -width ".Dv RA_UNLOCKED" 268.It Dv RA_LOCKED 269Assert that current thread holds either a shared or exclusive lock 270of 271.Fa rw . 272.It Dv RA_RLOCKED 273Assert that current thread holds a shared lock of 274.Fa rw . 275.It Dv RA_WLOCKED 276Assert that current thread holds an exclusive lock of 277.Fa rw . 278.It Dv RA_UNLOCKED 279Assert that current thread holds neither a shared nor exclusive lock of 280.Fa rw . 281.El 282.Pp 283In addition, one of the following optional flags may be specified with 284.Dv RA_LOCKED , 285.Dv RA_RLOCKED , 286or 287.Dv RA_WLOCKED : 288.Bl -tag -width ".Dv RA_NOTRECURSED" 289.It Dv RA_RECURSED 290Assert that the current thread holds a recursive lock of 291.Fa rw . 292.It Dv RA_NOTRECURSED 293Assert that the current thread does not hold a recursive lock of 294.Fa rw . 295.El 296.El 297.Sh SEE ALSO 298.Xr locking 9 , 299.Xr mutex 9 , 300.Xr panic 9 , 301.Xr sema 9 , 302.Xr sx 9 303.Sh HISTORY 304These 305functions appeared in 306.Fx 7.0 . 307.Sh AUTHORS 308.An -nosplit 309The 310.Nm 311facility was written by 312.An "John Baldwin" . 313This manual page was written by 314.An "Gleb Smirnoff" . 315.Sh BUGS 316A kernel without 317.Dv WITNESS 318cannot assert whether the current thread does or does not hold a read lock. 319.Dv RA_LOCKED 320and 321.Dv RA_RLOCKED 322can only assert that 323.Em any 324thread holds a read lock. 325They cannot ensure that the current thread holds a read lock. 326Further, 327.Dv RA_UNLOCKED 328can only assert that the current thread does not hold a write lock. 329.Pp 330Reader/writer is a bit of an awkward name. 331An 332.Nm 333can also be called a 334.Dq Robert Watson 335lock if desired. 336