xref: /dragonfly/lib/libc/sys/umtx.2 (revision 7bc7e232) 
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33.\" $DragonFly: src/lib/libc/sys/umtx.2,v 1.11 2007/07/23 23:04:21 dillon Exp $
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 simultaneously 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
78.Dx
79implementation optimized the count = 1 case but otherwise just wakes up
80all processes sleeping on the address.
81.Pp
82Kernel support for userland mutexes is based on the physical memory backing
83the user address.  Two userland programs may use this facility through
84.Fn mmap ,
85.Fn sysv ,
86.Fn rfork ,
87or light weight process-based shared memory.
88It is important to note that the kernel does not
89take responsibility for adjusting the contents of the mutex or for the
90userland implementation of the mutex.
91.Pp
92.Fn umtx_sleep
93does not restart in case of a signal, even if the signal specifies
94that system calls should restart.
95.Sh RETURN VALUES
96.Fn umtx_sleep
97will return 0 if it successfully slept and was then woken up.  Otherwise
98it will return -1 and set
99.Va errno
100as shown below.
101.Pp
102.Fn umtx_wakeup
103will generally return 0 unless the address is bad.
104.Sh EXAMPLE
105.Bd -literal -compact
106
107void
108userland_get_mutex(struct umtx *mtx)
109{
110    int v;
111
112    for (;;) {
113	v = mtx->lock;
114	if ((v & MTX_LOCKED) == 0) {
115	    /*
116	     * not locked, attempt to lock.
117	     */
118	    if (cmp_and_exg(&mtx->lock, v, v | MTX_LOCKED) == 0)
119		return;
120	} else {
121	    /*
122	     * Locked, bump the contested count and obtain the contested
123	     * mutex.
124	     */
125	    if (cmp_and_exg(&mtx->lock, v, v + 1) == 0) {
126		userland_get_mutex_contested(mtx);
127		return;
128	    }
129	}
130    }
131}
132
133static void
134userland_get_mutex_contested(struct umtx *mtx)
135{
136    int v;
137
138    for (;;) {
139	v = mtx->lock;
140	assert(v & ~MTX_LOCKED);	/* our contesting count still there */
141	if ((v & MTX_LOCKED) == 0) {
142	    /*
143	     * not locked, attempt to remove our contested count and
144	     * lock at the same time.
145	     */
146	    if (cmp_and_exg(&mtx->lock, v, (v - 1) | MTX_LOCKED) == 0)
147		return;
148	} else {
149	    /*
150	     * Still locked, sleep and try again.
151	     */
152	    umtx_sleep(&mtx->lock, v, 0);
153	    /*
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.
157	     */
158	}
159    }
160}
161
162void
163userland_rel_mutex(struct umtx *mtx)
164{
165    int v;
166
167    for (;;) {
168	v = mtx->lock;
169	assert(v & MTX_LOCKED);	/* we still have it locked */
170	if (v == MTX_LOCKED) {
171	    /*
172	     * We hold an uncontested lock, try to set to an unlocked
173	     * state.
174	     */
175	    if (cmp_and_exg(&mtx->lock, MTX_LOCKED, 0) == 0)
176		return;
177	} else {
178	    /*
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.
183	     */
184	    if (cmp_and_exg(&mtx->lock, v, v & ~MTX_LOCKED) == 0) {
185		umtx_wakeup(&mtx->lock, 1);
186		return;
187	    }
188	}
189    }
190}
191.Ed
192.Sh ERRORS
193.Bl -tag -width Er
194.It Bq Er EBUSY
195The contents of
196.Fa *ptr
197did not match
198.Fa value
199.It Bq Er ETIMEDOUT
200The specified timeout occurred.
201.It Bq Er EINTR
202The
203.Fn umtx_sleep
204call was interrupted by a signal.
205.It Bq Er EINVAL
206An invalid parameter (typically an invalid timeout) was specified.
207.El
208.Sh SEE ALSO
209.Xr tls 2
210.Sh HISTORY
211The
212.Fn umtx_sleep ,
213and
214.Fn umtx_wakeup
215function calls first appeared in
216.Dx 1.1 .
217