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33 .\" $DragonFly: src/lib/libc/sys/umtx.2,v 1.12 2008/04/14 20:17:41 dillon Exp $
41 .Nd kernel support for userland mutexes
47 .Fn umtx_sleep "const int *ptr" "int value" "int timeout"
49 .Fn umtx_wakeup "const int *ptr" "int count"
53 system call will put the calling process to sleep for
55 microseconds if the contents of the specified pointer matches
57 Specifying a timeout of 0 indicates an indefinite timeout.
58 The comparison is not atomic with the sleep but is properly
59 interlocked against another process calling
61 In particular, while it is possible for two userland threads to race, one
62 going to sleep simultaneously with another releasing the mutex, this condition
63 is caught when the second userland thread calls
65 after releasing the contended mutex.
69 has no specific limitation other than what fits in the signed integer.
70 A negative timeout will return
75 system call will wakeup the specified number of processes sleeping
78 on the specified user address. A count of 0 will wake up all sleeping
79 processes. This function may wake up more processes then the specified
80 count but will never wake up fewer processes (unless there are simply not
81 that many currently sleeping on the address). The current
83 implementation optimized the count = 1 case but otherwise just wakes up
84 all processes sleeping on the address.
86 Kernel support for userland mutexes is based on the physical memory backing
87 the user address. Two userland programs may use this facility through
91 or light weight process-based shared memory.
92 It is important to note that the kernel does not
93 take responsibility for adjusting the contents of the mutex or for the
94 userland implementation of the mutex.
97 does not restart in case of a signal, even if the signal specifies
98 that system calls should restart.
100 Various operating system events can cause
102 to return prematurely, with the contents of the mutex unchanged relative
103 to the compare value. Callers must be able to deal with such returns.
106 will return 0 if it successfully slept and was then woken up. Otherwise
107 it will return -1 and set
112 will generally return 0 unless the address is bad.
114 .Bd -literal -compact
117 userland_get_mutex(struct umtx *mtx)
123 if ((v & MTX_LOCKED) == 0) {
125 * not locked, attempt to lock.
127 if (cmp_and_exg(&mtx->lock, v, v | MTX_LOCKED) == 0)
131 * Locked, bump the contested count and obtain the contested
134 if (cmp_and_exg(&mtx->lock, v, v + 1) == 0) {
135 userland_get_mutex_contested(mtx);
143 userland_get_mutex_contested(struct umtx *mtx)
149 assert(v & ~MTX_LOCKED); /* our contesting count still there */
150 if ((v & MTX_LOCKED) == 0) {
152 * not locked, attempt to remove our contested count and
153 * lock at the same time.
155 if (cmp_and_exg(&mtx->lock, v, (v - 1) | MTX_LOCKED) == 0)
159 * Still locked, sleep and try again.
161 umtx_sleep(&mtx->lock, v, 0);
163 * XXX note: if we are woken up here but do not proceed to
164 * attempt to obtain the mutex, we should chain the
165 * umtx_wakeup() along.
172 userland_rel_mutex(struct umtx *mtx)
178 assert(v & MTX_LOCKED); /* we still have it locked */
179 if (v == MTX_LOCKED) {
181 * We hold an uncontested lock, try to set to an unlocked
184 if (cmp_and_exg(&mtx->lock, MTX_LOCKED, 0) == 0)
188 * We hold a contested lock, unlock and wakeup exactly
189 * one sleeper. It is possible for this to race a new
190 * thread obtaining a lock, in which case any contested
191 * sleeper we wake up will simply go back to sleep.
193 if (cmp_and_exg(&mtx->lock, v, v & ~MTX_LOCKED) == 0) {
194 umtx_wakeup(&mtx->lock, 1);
208 .It Bq Er EWOULDBLOCK
209 The specified timeout occurred.
213 call was interrupted by a signal.
215 An invalid parameter (typically an invalid timeout) was specified.
224 function calls first appeared in