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33 .\" $DragonFly: src/lib/libc/sys/umtx.2,v 1.11 2007/07/23 23:04:21 dillon Exp $
41 .Nd kernel support for userland mutexes
46 .Fn umtx_sleep "const int *ptr" "int value" "int timeout"
48 .Fn umtx_wakeup "const int *ptr" "int count"
52 system call will put the calling process to sleep for
54 microseconds if the contents of the specified point matches
56 Specifying a timeout of 0 indicates an indefinite timeout.
57 The comparison is not atomic with the sleep but is properly
58 interlocked against another process calling
60 In particular, while it is possible for two userland threads to race, one
61 going to sleep simultaneously with another releasing the mutex, this condition
62 is caught when the second userland thread calls
64 after releasing the contended mutex.
67 is limited to the range 0-1000000 microseconds.
71 system call will wakeup the specified number of processes sleeping
74 on the specified user address. A count of 0 will wake up all sleeping
75 processes. This function may wake up more processes then the specified
76 count but will never wake up fewer processes (unless there are simply not
77 that many currently sleeping on the address). The current
79 implementation optimized the count = 1 case but otherwise just wakes up
80 all processes sleeping on the address.
82 Kernel support for userland mutexes is based on the physical memory backing
83 the user address. Two userland programs may use this facility through
87 or light weight process-based shared memory.
88 It is important to note that the kernel does not
89 take responsibility for adjusting the contents of the mutex or for the
90 userland implementation of the mutex.
93 does not restart in case of a signal, even if the signal specifies
94 that system calls should restart.
97 will return 0 if it successfully slept and was then woken up. Otherwise
98 it will return -1 and set
103 will generally return 0 unless the address is bad.
105 .Bd -literal -compact
108 userland_get_mutex(struct umtx *mtx)
114 if ((v & MTX_LOCKED) == 0) {
116 * not locked, attempt to lock.
118 if (cmp_and_exg(&mtx->lock, v, v | MTX_LOCKED) == 0)
122 * Locked, bump the contested count and obtain the contested
125 if (cmp_and_exg(&mtx->lock, v, v + 1) == 0) {
126 userland_get_mutex_contested(mtx);
134 userland_get_mutex_contested(struct umtx *mtx)
140 assert(v & ~MTX_LOCKED); /* our contesting count still there */
141 if ((v & MTX_LOCKED) == 0) {
143 * not locked, attempt to remove our contested count and
144 * lock at the same time.
146 if (cmp_and_exg(&mtx->lock, v, (v - 1) | MTX_LOCKED) == 0)
150 * Still locked, sleep and try again.
152 umtx_sleep(&mtx->lock, v, 0);
154 * XXX note: if we are woken up here but do not proceed to
155 * attempt to obtain the mutex, we should chain the
156 * umtx_wakeup() along.
163 userland_rel_mutex(struct umtx *mtx)
169 assert(v & MTX_LOCKED); /* we still have it locked */
170 if (v == MTX_LOCKED) {
172 * We hold an uncontested lock, try to set to an unlocked
175 if (cmp_and_exg(&mtx->lock, MTX_LOCKED, 0) == 0)
179 * We hold a contested lock, unlock and wakeup exactly
180 * one sleeper. It is possible for this to race a new
181 * thread obtaining a lock, in which case any contested
182 * sleeper we wake up will simply go back to sleep.
184 if (cmp_and_exg(&mtx->lock, v, v & ~MTX_LOCKED) == 0) {
185 umtx_wakeup(&mtx->lock, 1);
200 The specified timeout occurred.
204 call was interrupted by a signal.
206 An invalid parameter (typically an invalid timeout) was specified.
215 function calls first appeared in