xref: /dragonfly/lib/libc/sys/umtx.2 (revision 3f5e28f4) 
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33.\" $DragonFly: src/lib/libc/sys/umtx.2,v 1.9 2007/05/17 08:19:00 swildner 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 ,
86and
87.Fn rfork
88based shared memory.  It 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.Sh RETURN VALUES
92.Fn umtx_sleep
93will return 0 if it successfully slept and was then woken up.  Otherwise
94it will return -1 and set
95.Li errno
96as shown below.
97.Pp
98.Fn umtx_wakeup
99will generally return 0 unless the address is bad.
100.Sh EXAMPLE
101.Bd -literal -compact
102
103void
104userland_get_mutex(struct umtx *mtx)
105{
106    int v;
107
108    for (;;) {
109	v = mtx->lock;
110	if ((v & MTX_LOCKED) == 0) {
111	    /*
112	     * not locked, attempt to lock.
113	     */
114	    if (cmp_and_exg(&mtx->lock, v, v | MTX_LOCKED) == 0)
115		return;
116	} else {
117	    /*
118	     * Locked, bump the contested count and obtain the contested
119	     * mutex.
120	     */
121	    if (cmp_and_exg(&mtx->lock, v, v + 1) == 0) {
122		userland_get_mutex_contested(mtx);
123		return;
124	    }
125	}
126    }
127}
128
129static void
130userland_get_mutex_contested(struct umtx *mtx)
131{
132    int v;
133
134    for (;;) {
135	v = mtx->lock;
136	assert(v & ~MTX_LOCKED);	/* our contesting count still there */
137	if ((v & MTX_LOCKED) == 0) {
138	    /*
139	     * not locked, attempt to remove our contested count and
140	     * lock at the same time.
141	     */
142	    if (cmp_and_exg(&mtx->lock, v, (v - 1) | MTX_LOCKED) == 0)
143		return;
144	} else {
145	    /*
146	     * Still locked, sleep and try again.
147	     */
148	    umtx_sleep(&mtx->lock, v, 0);
149	    /*
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.
153	     */
154	}
155    }
156}
157
158void
159userland_rel_mutex(struct umtx *mtx)
160{
161    int v;
162
163    for (;;) {
164	v = mtx->lock;
165	assert(v & MTX_LOCKED);	/* we still have it locked */
166	if (v == MTX_LOCKED) {
167	    /*
168	     * We hold an uncontested lock, try to set to an unlocked
169	     * state.
170	     */
171	    if (cmp_and_exg(&mtx->lock, MTX_LOCKED, 0) == 0)
172		return;
173	} else {
174	    /*
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.
179	     */
180	    if (cmp_and_exg(&mtx->lock, v, v & ~MTX_LOCKED) == 0) {
181		umtx_wakeup(&mtx->lock, 1);
182		return;
183	    }
184	}
185    }
186}
187.Ed
188.Sh ERRORS
189.Bl -tag -width Er
190.It Bq Er EBUSY
191The contents of
192.Fa *ptr
193did not match
194.Fa value
195.It Bq Er ETIMEDOUT
196The specified timeout occurred.
197.It Bq Er EINTR
198The
199.Fn umtx_sleep
200call was interrupted by a signal.
201.It Bq Er EINVAL
202An invalid parameter (typically an invalid timeout) was specified.
203.El
204.Sh SEE ALSO
205.Xr tls 2
206.Sh HISTORY
207The
208.Fn umtx_sleep ,
209and
210.Fn umtx_wakeup
211function calls first appeared in
212.Dx 1.1 .
213