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33 .\" $DragonFly: src/lib/libc/sys/umtx.2,v 1.3 2005/02/23 21:54:01 swildner 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 simultaniously 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 DragonFly
78 implementation optimized the count = 1 case but otherwise just wakes up
79 all processes sleeping on the address.
81 Kernel support for userland mutexes is based on the physical memory backing
82 the user address. Two userland programs may use this facility through
87 based shared memory. It is important to note that the kernel does not
88 take responsibility for adjusting the contents of the mutex or for the
89 userland implementation of the mutex.
92 will return 0 if it successfully slept and was then woken up. Otherwise
93 it will return -1 and set
98 will generally return 0 unless the address is bad.
107 The specified timeout occured.
111 call was interrupted by a signal.
113 An invalid parameter (typically an invalid timeout) was specified.
116 .Bd -literal -compact
119 userland_get_mutex(struct umtx *mtx)
125 if ((v & MTX_LOCKED) == 0) {
127 * not locked, attempt to lock.
129 if (cmp_and_exg(&mtx->lock, v, v | MTX_LOCKED) == 0)
133 * Locked, bump the contested count and obtain the contested
136 if (cmp_and_exg(&mtx->lock, v, v + 1) == 0) {
137 userland_get_mutex_contested(mtx);
145 userland_get_mutex_contested(struct umtx *mtx)
151 assert(v & ~MTX_LOCKED); /* our contesting count still there */
152 if ((v & MTX_LOCKED) == 0) {
154 * not locked, attempt to remove our contested count and
155 * lock at the same time.
157 if (cmp_and_exg(&mtx->lock, v, (v - 1) | MTX_LOCKED) == 0)
161 * Still locked, sleep and try again.
163 umtx_sleep(&mtx->lock, v, 0);
165 * XXX note: if we are woken up here but do not proceed to
166 * attempt to obtain the mutex, we should chain the
167 * umtx_wakeup() along.
174 userland_rel_mutex(struct umtx *mtx)
180 assert(v & MTX_LOCKED); /* we still have it locked */
181 if (v == MTX_LOCKED) {
183 * We hold an uncontested lock, try to set to an unlocked
186 if (cmp_and_exg(&mtx->lock, MTX_LOCKED, 0) == 0)
190 * We hold a contested lock, unlock and wakeup exactly
191 * one sleeper. It is possible for this to race a new
192 * thread obtaining a lock, in which case any contested
193 * sleeper we wake up will simply go back to sleep.
195 if (cmp_and_exg(&mtx->lock, v, v & ~MTX_LOCKED) == 0) {
196 umtx_wakeup(&mtx->lock, 1);
212 function calls first appeared in DragonFly 1.1 .