2 .\" Copyright (c) 1998 Berkeley Software Design, Inc. All rights reserved.
4 .\" Redistribution and use in source and binary forms, with or without
5 .\" modification, are permitted provided that the following conditions
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 .\" 3. Berkeley Software Design Inc's name may not be used to endorse or
13 .\" promote products derived from this software without specific prior
14 .\" written permission.
16 .\" THIS SOFTWARE IS PROVIDED BY BERKELEY SOFTWARE DESIGN INC ``AS IS'' AND
17 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
18 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
19 .\" ARE DISCLAIMED. IN NO EVENT SHALL BERKELEY SOFTWARE DESIGN INC BE LIABLE
20 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
21 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
22 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
23 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
24 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
25 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
28 .\" from BSDI $Id: mutex.4,v 1.1.2.3 1998/04/27 22:53:13 ewv Exp $
41 .Nm mtx_lock_spin_flags ,
43 .Nm mtx_trylock_flags ,
46 .Nm mtx_unlock_flags ,
47 .Nm mtx_unlock_spin_flags ,
54 .Nd kernel synchronization primitives
60 .Fn mtx_init "struct mtx *mutex" "const char *name" "const char *type" "int opts"
62 .Fn mtx_destroy "struct mtx *mutex"
64 .Fn mtx_lock "struct mtx *mutex"
66 .Fn mtx_lock_spin "struct mtx *mutex"
68 .Fn mtx_lock_flags "struct mtx *mutex" "int flags"
70 .Fn mtx_lock_spin_flags "struct mtx *mutex" "int flags"
72 .Fn mtx_trylock "struct mtx *mutex"
74 .Fn mtx_trylock_flags "struct mtx *mutex" "int flags"
76 .Fn mtx_unlock "struct mtx *mutex"
78 .Fn mtx_unlock_spin "struct mtx *mutex"
80 .Fn mtx_unlock_flags "struct mtx *mutex" "int flags"
82 .Fn mtx_unlock_spin_flags "struct mtx *mutex" "int flags"
84 .Fn mtx_sleep "void *chan" "struct mtx *mtx" "int priority" "const char *wmesg" "int timo"
86 .Fn mtx_initialized "struct mtx *mutex"
88 .Fn mtx_owned "struct mtx *mutex"
90 .Fn mtx_recursed "struct mtx *mutex"
92 .Cd "options INVARIANTS"
93 .Cd "options INVARIANT_SUPPORT"
95 .Fn mtx_assert "struct mtx *mutex" "int what"
97 .Fn MTX_SYSINIT "name" "struct mtx *mtx" "const char *description" "int opts"
99 Mutexes are the most basic and primary method of thread synchronization.
100 The major design considerations for mutexes are:
103 Acquiring and releasing uncontested mutexes should be as cheap
106 They must have the information and storage space to support
107 priority propagation.
109 A thread must be able to recursively acquire a mutex,
110 provided that the mutex is initialized to support recursion.
113 There are currently two flavors of mutexes, those that context switch
114 when they block and those that do not.
118 mutexes will context switch when they are already held.
120 they may spin for some amount
121 of time before context switching.
122 It is important to remember that since a thread may be preempted at any time,
123 the possible context switch introduced by acquiring a mutex is guaranteed
124 to not break anything that is not already broken.
126 Mutexes which do not context switch are
129 These should only be used to protect data shared with primary interrupt
131 This includes interrupt filters and low level scheduling code.
132 In all architectures both acquiring and releasing of a
133 uncontested spin mutex is more expensive than the same operation
135 In order to protect an interrupt service routine from blocking
136 against itself all interrupts are either blocked or deferred on a processor
137 while holding a spin lock.
138 It is permissible to hold multiple spin mutexes.
140 Once a spin mutex has been acquired it is not permissible to acquire a
143 The storage needed to implement a mutex is provided by a
145 In general this should be treated as an opaque object and
146 referenced only with the mutex primitives.
150 function must be used to initialize a mutex
151 before it can be passed to any of the other mutex functions.
154 option is used to identify the lock in debugging output etc.
157 option is used by the witness code to classify a mutex when doing checks
164 is used in its place.
165 The pointer passed in as
169 is saved rather than the data it points to.
170 The data pointed to must remain stable
171 until the mutex is destroyed.
174 argument is used to set the type of mutex.
175 It may contain either
180 See below for additional initialization options.
181 It is not permissible to pass the same
185 multiple times without intervening calls to
192 mutual exclusion lock
193 on behalf of the currently running kernel thread.
194 If another kernel thread is holding the mutex,
195 the caller will be disconnected from the CPU
196 until the mutex is available
197 (i.e., it will block).
203 mutual exclusion lock
204 on behalf of the currently running kernel thread.
205 If another kernel thread is holding the mutex,
206 the caller will spin until the mutex becomes available.
207 Interrupts are disabled during the spin and remain disabled
208 following the acquiring of the lock.
210 It is possible for the same thread to recursively acquire a mutex
211 with no ill effects, provided that the
215 during the initialization of the mutex.
220 .Fn mtx_lock_spin_flags
225 lock, respectively, and also accept a
228 In both cases, the only flag presently available for lock acquires is
232 bit is turned on in the
236 tracing is being done,
237 it will be silenced during the lock acquire.
241 attempts to acquire the
245 If the mutex cannot be immediately acquired
248 otherwise the mutex will be acquired
249 and a non-zero value will be returned.
252 .Fn mtx_trylock_flags
253 function has the same behavior as
255 but should be used when the caller desires to pass in a
258 Presently, the only valid value in the
262 and its effects are identical to those described for
270 mutual exclusion lock.
271 The current thread may be preempted if a higher priority thread is waiting
278 mutual exclusion lock.
283 .Fn mtx_unlock_spin_flags
284 functions behave in exactly the same way as do the standard mutex
285 unlock routines above, while also allowing a
287 argument which may specify
291 is identical to its behavior in the mutex lock routines.
295 function is used to destroy
297 so the data associated with it may be freed
298 or otherwise overwritten.
299 Any mutex which is destroyed
300 must previously have been initialized with
302 It is permissible to have a single hold count
303 on a mutex when it is destroyed.
304 It is not permissible to hold the mutex recursively,
305 or have another thread blocked on the mutex
306 when it is destroyed.
310 function is used to atomically release
312 while waiting for an event.
313 For more details on the parameters to this function,
319 function returns non-zero if
321 has been initialized and zero otherwise.
325 function returns non-zero
326 if the current thread holds
328 If the current thread does not hold
334 function returns non-zero if the
337 This check should only be made if the running thread already owns
342 function allows assertions specified in
346 If the assertions are not true and the kernel is compiled with
347 .Cd "options INVARIANTS"
349 .Cd "options INVARIANT_SUPPORT" ,
350 the kernel will panic.
351 Currently the following assertions are supported:
352 .Bl -tag -width MA_NOTRECURSED
354 Assert that the current thread
356 pointed to by the first argument.
358 Assert that the current thread
359 does not hold the mutex
360 pointed to by the first argument.
362 Assert that the current thread has recursed on the mutex
363 pointed to by the first argument.
364 This assertion is only valid in conjunction with
366 .It Dv MA_NOTRECURSED
367 Assert that the current thread has not recursed on the mutex
368 pointed to by the first argument.
369 This assertion is only valid in conjunction with
375 macro is used to generate a call to the
377 routine at system startup in order to initialize a given mutex lock.
378 The parameters are the same as
380 but with an additional argument,
382 that is used in generating unique variable names for the related structures associated with the lock and the sysinit routine.
383 .Ss The Default Mutex Type
384 Most kernel code should use the default lock type,
386 The default lock type will allow the thread
387 to be disconnected from the CPU
388 if the lock is already held by another thread.
390 may treat the lock as a short term spin lock
391 under some circumstances.
392 However, it is always safe to use these forms of locks
393 in an interrupt thread
394 without fear of deadlock
395 against an interrupted thread on the same CPU.
396 .Ss The Spin Mutex Type
399 mutex will not relinquish the CPU
400 when it cannot immediately get the requested lock,
401 but will loop, waiting for the mutex to be released by another CPU.
402 This could result in deadlock
403 if another thread interrupted the thread which held a mutex
404 and then tried to acquire the mutex.
405 For this reason spin locks disable all interrupts on the local CPU.
407 Spin locks are fairly specialized locks
408 that are intended to be held for very short periods of time.
409 Their primary purpose is to protect portions of the code
410 that implement other synchronization primitives such as default mutexes,
411 thread scheduling, and interrupt threads.
412 .Ss Initialization Options
413 The options passed in the
417 specify the mutex type.
422 options is required and only one of those two options may be specified.
423 The possibilities are:
424 .Bl -tag -width MTX_NOWITNESS
427 will always allow the current thread to be suspended
428 to avoid deadlock conditions against interrupt threads.
429 The implementation of this lock type
430 may spin for a while before suspending the current thread.
433 will never relinquish the CPU.
434 All interrupts are disabled on the local CPU
435 while any spin lock is held.
437 Specifies that the initialized mutex is allowed to recurse.
438 This bit must be present if the mutex is permitted to recurse.
440 Do not log any mutex operations for this lock.
446 Witness should not log messages about duplicate locks being acquired.
448 Do not profile this lock.
450 .Ss Lock and Unlock Flags
451 The flags passed to the
453 .Fn mtx_lock_spin_flags ,
454 .Fn mtx_unlock_flags ,
456 .Fn mtx_unlock_spin_flags
457 functions provide some basic options to the caller,
458 and are often used only under special circumstances to modify lock or
460 Standard locking and unlocking should be performed with the
467 Only if a flag is required should the corresponding
468 flags-accepting routines be used.
470 Options that modify mutex behavior:
471 .Bl -tag -width MTX_QUIET
473 This option is used to quiet logging messages during individual mutex
475 This can be used to trim superfluous logging messages for debugging purposes.
480 must be acquired, it must be acquired prior to acquiring
482 Put another way: it is impossible to acquire
484 non-recursively while
485 holding another mutex.
486 It is possible to acquire other mutexes while holding
488 and it is possible to acquire
490 recursively while holding other mutexes.
492 Sleeping while holding a mutex (except for
495 and should be avoided.
496 There are numerous assertions which will fail if this is attempted.
497 .Ss Functions Which Access Memory in Userspace
498 No mutexes should be held (except for
500 across functions which
501 access memory in userspace, such as
507 No locks are needed when calling these functions.
510 .Xr LOCK_PROFILING 9 ,
520 functions appeared in