1.\" $NetBSD: condvar.9,v 1.20 2019/04/12 11:02:22 abhinav Exp $ 2.\" 3.\" Copyright (c) 2006, 2007, 2008 The NetBSD Foundation, Inc. 4.\" All rights reserved. 5.\" 6.\" This code is derived from software contributed to The NetBSD Foundation 7.\" by Andrew Doran. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 19.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 20.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 21.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 22.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 23.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 24.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 25.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 26.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 27.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 28.\" POSSIBILITY OF SUCH DAMAGE. 29.\" 30.Dd November 12, 2017 31.Dt CONDVAR 9 32.Os 33.Sh NAME 34.Nm cv , 35.Nm condvar , 36.Nm cv_init , 37.Nm cv_destroy , 38.Nm cv_wait , 39.Nm cv_wait_sig , 40.Nm cv_timedwait , 41.Nm cv_timedwait_sig , 42.Nm cv_timedwaitbt , 43.Nm cv_timedwaitbt_sig , 44.Nm cv_signal , 45.Nm cv_broadcast , 46.Nm cv_has_waiters 47.Nd condition variables 48.Sh SYNOPSIS 49.In sys/condvar.h 50.Ft void 51.Fn cv_init "kcondvar_t *cv" "const char *wmesg" 52.Ft void 53.Fn cv_destroy "kcondvar_t *cv" 54.Ft void 55.Fn cv_wait "kcondvar_t *cv" "kmutex_t *mtx" 56.Ft int 57.Fn cv_wait_sig "kcondvar_t *cv" "kmutex_t *mtx" 58.Ft int 59.Fn cv_timedwait "kcondvar_t *cv" "kmutex_t *mtx" "int ticks" 60.Ft int 61.Fn cv_timedwait_sig "kcondvar_t *cv" "kmutex_t *mtx" "int ticks" 62.Ft int 63.Fn cv_timedwaitbt "kcondvar_t *cv" "kmutex_t *mtx" "struct bintime *bt" \ 64"const struct bintime *epsilon" 65.Ft int 66.Fn cv_timedwaitbt_sig "kcondvar_t *cv" "kmutex_t *mtx" "struct bintime *bt" \ 67"const struct bintime *epsilon" 68.Ft void 69.Fn cv_signal "kcondvar_t *cv" 70.Ft void 71.Fn cv_broadcast "kcondvar_t *cv" 72.Ft bool 73.Fn cv_has_waiters "kcondvar_t *cv" 74.Pp 75.Cd "options DIAGNOSTIC" 76.Cd "options LOCKDEBUG" 77.Sh DESCRIPTION 78Condition variables (CVs) are used in the kernel to synchronize access 79to resources that are limited (for example, memory) and to wait for 80pending I/O operations to complete. 81.Pp 82The 83.Vt kcondvar_t 84type provides storage for the CV object. 85This should be treated as an opaque object and not examined directly by 86consumers. 87.Sh OPTIONS 88.Bl -tag -width abcd 89.It Cd "options DIAGNOSTIC" 90.Pp 91Kernels compiled with the 92.Dv DIAGNOSTIC 93option perform basic sanity checks on CV operations. 94.It Cd "options LOCKDEBUG" 95.Pp 96Kernels compiled with the 97.Dv LOCKDEBUG 98option perform potentially CPU intensive sanity checks 99on CV operations. 100.El 101.Sh FUNCTIONS 102.Bl -tag -width abcd 103.It Fn cv_init "cv" "wmesg" 104.Pp 105Initialize a CV for use. 106No other operations can be performed on the CV until it has been initialized. 107.Pp 108The 109.Fa wmesg 110argument specifies a string of no more than 8 characters that describes 111the resource or condition associated with the CV. 112The kernel does not use this argument directly but makes it available for 113utilities such as 114.Xr ps 1 115to display. 116.It Fn cv_destroy "cv" 117.Pp 118Release resources used by a CV. 119The CV must not be in use when it is destroyed, and must not be used afterwards. 120.It Fn cv_wait "cv" "mtx" 121.Pp 122Cause the current LWP to wait non-interruptably for access to a resource, 123or for an I/O operation to complete. 124The LWP will resume execution when awoken by another thread using 125.Fn cv_signal 126or 127.Fn cv_broadcast . 128.Pp 129.Fa mtx 130specifies a kernel mutex to be used as an interlock, and must be held by the 131calling LWP on entry to 132.Fn cv_wait . 133It will be released once the LWP has prepared to sleep, and will be reacquired 134before 135.Fn cv_wait 136returns. 137.Pp 138A small window exists between testing for availability of a resource and 139waiting for the resource with 140.Fn cv_wait , 141in which the resource may become available again. 142The interlock is used to guarantee that the resource will not be signalled 143as available until the calling LWP has begun to wait for it. 144.Pp 145Non-interruptable waits have the potential to deadlock the system, and so must 146be kept short (typically, under one second). 147.It Fn cv_wait_sig "cv" "mtx" 148.Pp 149As per 150.Fn cv_wait , 151but causes the current LWP to wait interruptably. 152If the LWP receives a signal, or is interrupted by another condition such 153as its containing process exiting, the wait is ended early and an error 154code returned. 155.Pp 156If 157.Fn cv_wait_sig 158returns as a result of a signal, the return value is 159.Er ERESTART 160if the signal 161has the 162.Dv SA_RESTART 163property. 164If awoken normally, the value is zero, and 165.Er EINTR 166under all other conditions. 167.It Fn cv_timedwait "cv" "mtx" "ticks" 168.Pp 169As per 170.Fn cv_wait , 171but will return early if a timeout specified by the 172.Fa ticks 173argument expires. 174.Pp 175.Fa ticks 176is an architecture and system dependent value related to the number of 177clock interrupts per second. 178See 179.Xr hz 9 180for details. 181The 182.Xr mstohz 9 183macro can be used to convert a timeout expressed in milliseconds to 184one suitable for 185.Fn cv_timedwait . 186If the 187.Fa ticks 188argument is zero, 189.Fn cv_timedwait 190behaves exactly like 191.Fn cv_wait . 192.Pp 193If the timeout expires before the LWP is awoken, the return value is 194.Er EWOULDBLOCK . 195If awoken normally, the return value is zero. 196.It Fn cv_timedwait_sig "cv" "mtx" "ticks" 197.Pp 198As per 199.Fn cv_wait_sig , 200but also accepts a timeout value and will return 201.Er EWOULDBLOCK 202if the timeout expires. 203.It Fn cv_timedwaitbt "cv" "mtx" "bt" "epsilon" 204.It Fn cv_timedwaitbt_sig "cv" "mtx" "bt" "epsilon" 205.Pp 206Similar to 207.Fn cv_timedwait 208and 209.Fn cv_timedwait_sig , 210but 211.Fa bt 212is decremented in place with the amount of time actually waited, and on 213return contains the amount of time remaining, possibly negative if the 214timeout expired. 215.Pp 216The hint 217.Fa epsilon 218requests that the wakeup not be delayed more than 219.Fa bt Li "+" Fa epsilon , 220so that the system can coalesce multiple wakeups within their 221respective epsilons into a single high-resolution clock interrupt or 222choose to use cheaper low-resolution clock interrupts instead. 223.Pp 224However, the system is still limited by its best clock interrupt 225resolution and by scheduling competition, which may delay the wakeup by 226more than 227.Fa bt Li "+" Fa epsilon . 228.It Fn cv_signal "cv" 229.Pp 230Awaken one LWP (potentially among many) that is waiting on the specified 231condition variable. 232The mutex passed to the wait function 233.Po Fa mtx Pc 234must also be held when calling 235.Fn cv_signal . 236.Pp 237(Note that 238.Fn cv_signal 239is erroneously named in that it does not send a signal in the traditional 240sense to LWPs waiting on a CV.) 241.It Fn cv_broadcast "cv" 242.Pp 243Awaken all LWPs waiting on the specified condition variable. 244The mutex passed to the wait function 245.Po Fa mtx Pc 246must also be held when calling 247.Fn cv_broadcast . 248.It Fn cv_has_waiters "cv" 249.Pp 250Return 251.Dv true 252if one or more LWPs are waiting on the specified condition variable. 253.Pp 254.Fn cv_has_waiters 255cannot test reliably for interruptable waits. 256It should only be used to test for non-interruptable waits 257made using 258.Fn cv_wait . 259.Pp 260.Fn cv_has_waiters 261should only be used when making diagnostic assertions, and must 262be called while holding the interlocking mutex passed to 263.Fn cv_wait . 264.El 265.Sh EXAMPLES 266Consuming a resource: 267.Bd -literal 268 /* 269 * Lock the resource. Its mutex will also serve as the 270 * interlock. 271 */ 272 mutex_enter(&res->mutex); 273 274 /* 275 * Wait for the resource to become available. Timeout after 276 * five seconds. If the resource is not available within the 277 * alloted time, return an error. 278 */ 279 struct bintime timeout = { .sec = 5, .frac = 0 }; 280 const struct bintime epsilon = { .sec = 1, .frac = 0 }; 281 while (res->state == BUSY) { 282 error = cv_timedwaitbt(&res->condvar, \\ 283 &res->mutex, &timeout, &epsilon); 284 if (error) { 285 KASSERT(error == EWOULDBLOCK); 286 if (res->state != BUSY) 287 break; 288 mutex_exit(&res->mutex); 289 return ETIMEDOUT; 290 } 291 } 292 293 /* 294 * It's now available to us. Take ownership of the 295 * resource, and consume it. 296 */ 297 res->state = BUSY; 298 mutex_exit(&res->mutex); 299 consume(res); 300.Ed 301.Pp 302Releasing a resource for the next consumer to use: 303.Bd -literal 304 mutex_enter(&res->mutex); 305 res->state = IDLE; 306 cv_signal(&res->condvar); 307 mutex_exit(&res->mutex); 308.Ed 309.Sh CODE REFERENCES 310The core of the CV implementation is in 311.Pa sys/kern/kern_condvar.c . 312.Pp 313The header file 314.Pa sys/sys/condvar.h 315describes the public interface. 316.Sh SEE ALSO 317.Xr sigaction 2 , 318.Xr membar_ops 3 , 319.Xr errno 9 , 320.Xr mstohz 9 , 321.Xr mutex 9 , 322.Xr rwlock 9 323.Pp 324.Rs 325.%A Jim Mauro 326.%A Richard McDougall 327.%T Solaris Internals: Core Kernel Architecture 328.%I Prentice Hall 329.%D 2001 330.%O ISBN 0-13-022496-0 331.Re 332.Sh HISTORY 333The CV primitives first appeared in 334.Nx 5.0 . 335The 336.Fn cv_timedwaitbt 337and 338.Fn cv_timedwaitbt_sig 339primitives first appeared in 340.Nx 9.0 . 341