1 .\" Copyright (c) 2003,2004 The DragonFly Project. All rights reserved.
3 .\" This code is derived from software contributed to The DragonFly Project
4 .\" by Matthew Dillon <dillon@backplane.com>
6 .\" Redistribution and use in source and binary forms, with or without
7 .\" modification, are permitted provided that the following conditions
10 .\" 1. Redistributions of source code must retain the above copyright
11 .\" notice, this list of conditions and the following disclaimer.
12 .\" 2. Redistributions in binary form must reproduce the above copyright
13 .\" notice, this list of conditions and the following disclaimer in
14 .\" the documentation and/or other materials provided with the
16 .\" 3. Neither the name of The DragonFly Project nor the names of its
17 .\" contributors may be used to endorse or promote products derived
18 .\" from this software without specific, prior written permission.
20 .\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
21 .\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
22 .\" LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
23 .\" FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
24 .\" COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
25 .\" INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
26 .\" BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
27 .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
28 .\" AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
29 .\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
30 .\" OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
33 .\" $DragonFly: src/lib/libc/sys/umtx.2,v 1.9 2007/05/17 08:19:00 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 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
88 based shared memory. 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 will return 0 if it successfully slept and was then woken up. Otherwise
94 it will return -1 and set
99 will generally return 0 unless the address is bad.
101 .Bd -literal -compact
104 userland_get_mutex(struct umtx *mtx)
110 if ((v & MTX_LOCKED) == 0) {
112 * not locked, attempt to lock.
114 if (cmp_and_exg(&mtx->lock, v, v | MTX_LOCKED) == 0)
118 * Locked, bump the contested count and obtain the contested
121 if (cmp_and_exg(&mtx->lock, v, v + 1) == 0) {
122 userland_get_mutex_contested(mtx);
130 userland_get_mutex_contested(struct umtx *mtx)
136 assert(v & ~MTX_LOCKED); /* our contesting count still there */
137 if ((v & MTX_LOCKED) == 0) {
139 * not locked, attempt to remove our contested count and
140 * lock at the same time.
142 if (cmp_and_exg(&mtx->lock, v, (v - 1) | MTX_LOCKED) == 0)
146 * Still locked, sleep and try again.
148 umtx_sleep(&mtx->lock, v, 0);
150 * XXX note: if we are woken up here but do not proceed to
151 * attempt to obtain the mutex, we should chain the
152 * umtx_wakeup() along.
159 userland_rel_mutex(struct umtx *mtx)
165 assert(v & MTX_LOCKED); /* we still have it locked */
166 if (v == MTX_LOCKED) {
168 * We hold an uncontested lock, try to set to an unlocked
171 if (cmp_and_exg(&mtx->lock, MTX_LOCKED, 0) == 0)
175 * We hold a contested lock, unlock and wakeup exactly
176 * one sleeper. It is possible for this to race a new
177 * thread obtaining a lock, in which case any contested
178 * sleeper we wake up will simply go back to sleep.
180 if (cmp_and_exg(&mtx->lock, v, v & ~MTX_LOCKED) == 0) {
181 umtx_wakeup(&mtx->lock, 1);
196 The specified timeout occurred.
200 call was interrupted by a signal.
202 An invalid parameter (typically an invalid timeout) was specified.
211 function calls first appeared in