Fix some groff warnings.
[dragonfly.git] / lib / libc / sys / umtx.2
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1.\" Copyright (c) 2003,2004 The DragonFly Project. All rights reserved.
2.\"
3.\" This code is derived from software contributed to The DragonFly Project
4.\" by Matthew Dillon <dillon@backplane.com>
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cb65615c 33.\" $DragonFly: src/lib/libc/sys/umtx.2,v 1.4 2005/03/08 20:29:55 swildner Exp $
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34.\"
35.Dd February 21, 2005
36.Dt UMTX 2
37.Os
38.Sh NAME
39.Nm umtx_sleep ,
40.Nm umtx_wakeup
41.Nd kernel support for userland mutexes
42.Sh LIBRARY
43.Lb libc
44.Sh SYNOPSIS
45.Ft int
46.Fn umtx_sleep "const int *ptr" "int value" "int timeout"
47.Ft int
48.Fn umtx_wakeup "const int *ptr" "int count"
49.Sh DESCRIPTION
50The
51.Fn umtx_sleep
52system call will put the calling process to sleep for
53.Fa timeout
54microseconds if the contents of the specified point matches
55the specified value.
56Specifying a timeout of 0 indicates an indefinite timeout.
57The comparison is not atomic with the sleep but is properly
58interlocked against another process calling
59.Fn umtx_wakeup .
60In particular, while it is possible for two userland threads to race, one
61going to sleep simultaniously with another releasing the mutex, this condition
62is caught when the second userland thread calls
63.Fn umtx_wakeup
64after releasing the contended mutex.
65The
66.Fa timeout
67is limited to the range 0-1000000 microseconds.
68.Pp
69The
70.Fn umtx_wakeup
71system call will wakeup the specified number of processes sleeping
72in
73.Fn umtx_sleep
74on the specified user address. A count of 0 will wake up all sleeping
75processes. This function may wake up more processes then the specified
76count but will never wake up fewer processes (unless there are simply not
77that many currently sleeping on the address). The current DragonFly
78implementation optimized the count = 1 case but otherwise just wakes up
79all processes sleeping on the address.
80.Pp
81Kernel support for userland mutexes is based on the physical memory backing
82the user address. Two userland programs may use this facility through
83.Fn mmap ,
84.Fn sysv ,
85and
86.Fn rfork
87based shared memory. It is important to note that the kernel does not
88take responsibility for adjusting the contents of the mutex or for the
89userland implementation of the mutex.
90.Sh RETURN VALUES
91.Fn umtx_sleep
92will return 0 if it successfully slept and was then woken up. Otherwise
93it will return -1 and set
94.Li errno
95as shown below.
96.Pp
97.Fn umtx_wakeup
98will generally return 0 unless the address is bad.
99.Sh ERRORS
100.Bl -tag -width Er
101.It Bq Er EBUSY
102The contents of
103.Fa *ptr
104did not match
105.Fa value
106.It Bq Er ETIMEDOUT
107The specified timeout occured.
108.It Bq Er EINTR
109The
110.Fn umtx_sleep
111call was interrupted by a signal.
112.It Bq Er EINVAL
113An invalid parameter (typically an invalid timeout) was specified.
720180cd 114.El
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115.Sh EXAMPLE
116.Bd -literal -compact
117
118void
119userland_get_mutex(struct umtx *mtx)
120{
121 int v;
122
123 for (;;) {
124 v = mtx->lock;
125 if ((v & MTX_LOCKED) == 0) {
126 /*
127 * not locked, attempt to lock.
128 */
129 if (cmp_and_exg(&mtx->lock, v, v | MTX_LOCKED) == 0)
130 return;
131 } else {
132 /*
133 * Locked, bump the contested count and obtain the contested
134 * mutex.
135 */
136 if (cmp_and_exg(&mtx->lock, v, v + 1) == 0) {
137 userland_get_mutex_contested(mtx);
138 return;
139 }
140 }
141 }
142}
143
144static void
145userland_get_mutex_contested(struct umtx *mtx)
146{
147 int v;
148
149 for (;;) {
150 v = mtx->lock;
151 assert(v & ~MTX_LOCKED); /* our contesting count still there */
152 if ((v & MTX_LOCKED) == 0) {
153 /*
154 * not locked, attempt to remove our contested count and
155 * lock at the same time.
156 */
157 if (cmp_and_exg(&mtx->lock, v, (v - 1) | MTX_LOCKED) == 0)
158 return;
159 } else {
160 /*
161 * Still locked, sleep and try again.
162 */
163 umtx_sleep(&mtx->lock, v, 0);
164 /*
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.
168 */
169 }
170 }
171}
172
173void
174userland_rel_mutex(struct umtx *mtx)
175{
176 int v;
177
178 for (;;) {
179 v = mtx->lock;
180 assert(v & MTX_LOCKED); /* we still have it locked */
181 if (v == MTX_LOCKED) {
182 /*
183 * We hold an uncontested lock, try to set to an unlocked
184 * state.
185 */
186 if (cmp_and_exg(&mtx->lock, MTX_LOCKED, 0) == 0)
187 return;
188 } else {
189 /*
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.
194 */
195 if (cmp_and_exg(&mtx->lock, v, v & ~MTX_LOCKED) == 0) {
196 umtx_wakeup(&mtx->lock, 1);
197 return;
198 }
199 }
200 }
201}
202.Ed
791e9ddb 203.Sh SEE ALSO
806bf111 204.Xr tls 2
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205.Sh HISTORY
206The
207.Fn umtx_sleep ,
208and
209.Fn umtx_wakeup
210function calls first appeared in DragonFly 1.1 .