xref: /freebsd/share/man/man9/sleepqueue.9 (revision 1e413cf93298b5b97441a21d9a50fdcd0ee9945e)
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24.\" $FreeBSD$
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26.Dd August 13, 2007
27.Dt SLEEPQUEUE 9
28.Os
29.Sh NAME
30.Nm init_sleepqueues ,
31.Nm sleepq_abort ,
32.Nm sleepq_add ,
33.Nm sleepq_alloc ,
34.Nm sleepq_broadcast ,
35.Nm sleepq_calc_signal_retval ,
36.Nm sleepq_catch_signals ,
37.Nm sleepq_free ,
38.Nm sleepq_lock ,
39.Nm sleepq_lookup ,
40.Nm sleepq_release ,
41.Nm sleepq_remove ,
42.Nm sleepq_signal ,
43.Nm sleepq_set_timeout ,
44.Nm sleepq_timedwait ,
45.Nm sleepq_timedwait_sig ,
46.Nm sleepq_wait ,
47.Nm sleepq_wait_sig
48.Nd manage the queues of sleeping threads
49.Sh SYNOPSIS
50.In sys/param.h
51.In sys/sleepqueue.h
52.Ft void
53.Fn init_sleepqueues "void"
54.Ft void
55.Fn sleepq_abort "struct thread *td"
56.Ft void
57.Fn sleepq_add "void *wchan" "struct lock_object *lock" "const char *wmesg" "int flags" "int queue"
58.Ft struct sleepqueue *
59.Fn sleepq_alloc "void"
60.Ft void
61.Fn sleepq_broadcast "void *wchan" "int flags" "int pri" "int queue"
62.Ft int
63.Fn sleepq_calc_signal_retval "int sig"
64.Ft int
65.Fn sleepq_catch_signals "void *wchan"
66.Ft void
67.Fn sleepq_free "struct sleepqueue *sq"
68.Ft struct sleepqueue *
69.Fn sleepq_lookup "void *wchan"
70.Ft void
71.Fn sleepq_lock "void *wchan"
72.Ft void
73.Fn sleepq_release "void *wchan"
74.Ft void
75.Fn sleepq_remove "struct thread *td" "void *wchan"
76.Ft void
77.Fn sleepq_signal "void *wchan" "int flags" "int pri" "int queue"
78.Ft void
79.Fn sleepq_set_timeout "void *wchan" "int timo"
80.Ft int
81.Fn sleepq_timedwait "void *wchan"
82.Ft int
83.Fn sleepq_timedwait_sig "void *wchan" "int signal_caught"
84.Ft void
85.Fn sleepq_wait "void *wchan"
86.Ft int
87.Fn sleepq_wait_sig "void *wchan"
88.Sh DESCRIPTION
89Sleep queues provide a mechanism for suspending execution of a thread until
90some condition is met.
91Each queue is associated with a specific wait channel when it is active,
92and only one queue may be associated with a wait channel at any given point
93in time.
94The implementation of each wait channel splits its sleepqueue into 2 sub-queues
95in order to enable some optimizations on threads' wakeups.
96An active queue holds a list of threads that are blocked on the associated
97wait channel.
98Threads that are not blocked on a wait channel have an associated inactive
99sleep queue.
100When a thread blocks on a wait channel it donates its inactive sleep queue
101to the wait channel.
102When a thread is resumed,
103the wait channel that it was blocked on gives it an inactive sleep queue for
104later use.
105.Pp
106The
107.Fn sleepq_alloc
108function allocates an inactive sleep queue and is used to assign a
109sleep queue to a thread during thread creation.
110The
111.Fn sleepq_free
112function frees the resources associated with an inactive sleep queue and is
113used to free a queue during thread destruction.
114.Pp
115Active sleep queues are stored in a hash table hashed on the addresses pointed
116to by wait channels.
117Each bucket in the hash table contains a sleep queue chain.
118A sleep queue chain contains a spin mutex and a list of sleep queues that hash
119to that specific chain.
120Active sleep queues are protected by their chain's spin mutex.
121The
122.Fn init_sleepqueues
123function initializes the hash table of sleep queue chains.
124.Pp
125The
126.Fn sleepq_lock
127function locks the sleep queue chain associated with wait channel
128.Fa wchan .
129.Pp
130The
131.Fn sleepq_lookup
132returns a pointer to the currently active sleep queue for that wait
133channel associated with
134.Fa wchan
135or
136.Dv NULL
137if there is no active sleep queue associated with
138argument
139.Fa wchan .
140It requires the sleep queue chain associated with
141.Fa wchan
142to have been locked by a prior call to
143.Fn sleepq_lock .
144.Pp
145The
146.Fn sleepq_release
147function unlocks the sleep queue chain associated with
148.Fn wchan
149and is primarily useful when aborting a pending sleep request before one of
150the wait functions is called.
151.Pp
152The
153.Fn sleepq_add
154function places the current thread on the sleep queue associated with the
155wait channel
156.Fa wchan .
157The sleep queue chain associated with argument
158.Fa wchan
159must be locked by a prior call to
160.Fn sleepq_lock
161when this function is called.
162If a lock is specified via the
163.Fa lock
164argument, and if the kernel was compiled with
165.Cd "options INVARIANTS" ,
166then the sleep queue code will perform extra checks to ensure that
167the lock is used by all threads sleeping on
168.Fa wchan .
169The
170.Fa wmesg
171parameter should be a short description of
172.Fa wchan .
173The
174.Fa flags
175parameter is a bitmask consisting of the type of sleep queue being slept on
176and zero or more optional flags.
177The
178.Fa queue
179parameter specifies the sub-queue, in which the contending thread will be
180inserted.
181.Pp
182There are currently three types of sleep queues:
183.Pp
184.Bl -tag -width ".Dv SLEEPQ_CONDVAR" -compact
185.It Dv SLEEPQ_CONDVAR
186A sleep queue used to implement condition variables.
187.It Dv SLEEPQ_SLEEP
188A sleep queue used to implement
189.Xr sleep 9 ,
190.Xr wakeup 9
191and
192.Xr wakeup_one 9 .
193.It Dv SLEEPQ_PAUSE
194A sleep queue used to implement
195.Xr pause 9 .
196.El
197.Pp
198There is currently only one optional flag:
199.Pp
200.Bl -tag -width ".Dv SLEEPQ_INTERRUPTIBLE" -compact
201.It Dv SLEEPQ_INTERRUPTIBLE
202The current thread is entering an interruptible sleep.
203.El
204.Pp
205A timeout on the sleep may be specified by calling
206.Fn sleepq_set_timeout
207after
208.Fn sleepq_add .
209The
210.Fa wchan
211parameter should be the same value from the preceding call to
212.Fn sleepq_add ,
213and the sleep queue chain associated with
214.Fa wchan
215must have been locked by a prior call to
216.Fn sleepq_lock .
217The
218.Fa timo
219parameter should specify the timeout value in ticks.
220.Pp
221The current thread may be marked interruptible by calling
222.Fn sleepq_catch_signals
223with
224.Fa wchan
225set to the wait channel.
226This function returns a signal number if there are any pending signals for
227the current thread and 0 if there is not a pending signal.
228The sleep queue chain associated with argument
229.Fa wchan
230should have been locked by a prior call to
231.Fn sleepq_lock .
232.Pp
233Once the thread is ready to suspend,
234one of the wait functions is called to put the current thread to sleep
235until it is awakened and to context switch to another thread.
236The
237.Fn sleepq_wait
238function is used for non-interruptible sleeps that do not have a timeout.
239The
240.Fn sleepq_timedwait
241function is used for non-interruptible sleeps that have had a timeout set via
242.Fn sleepq_set_timeout .
243The
244.Fn sleepq_wait_sig
245function is used for interruptible sleeps that do not have a timeout.
246The
247.Fn sleepq_timedwait_sig
248function is used for interruptible sleeps that do have a timeout set.
249The
250.Fa wchan
251argument to all of the wait functions is the wait channel being slept
252on.
253The sleep queue chain associated with argument
254.Fa wchan
255needs to have been locked with a prior call to
256.Fn sleepq_lock .
257The
258.Fa signal_caught
259parameter to
260.Fn sleepq_timedwait_sig
261specifies if a previous call to
262.Fn sleepq_catch_signals
263found a pending signal.
264.Pp
265When the thread is resumed,
266the wait functions return a non-zero value if the thread was awakened due to
267an interrupt other than a signal or a timeout.
268If the sleep timed out, then
269.Er EWOULDBLOCK
270is returned.
271If the sleep was interrupted by something other than a signal,
272then some other return value will be returned.
273If zero is returned after resuming from an interruptible sleep,
274then
275.Fn sleepq_calc_signal_retval
276should be called to determine if the sleep was interrupted by a signal.
277If so,
278.Fn sleepq_calc_signal_retval
279returns
280.Er ERESTART
281if the interrupting signal is restartable and
282.Er EINTR
283otherwise.
284If the sleep was not interrupted by a signal,
285.Fn sleepq_calc_signal_retval
286will return 0.
287.Pp
288A sleeping thread is normally resumed by the
289.Fn sleepq_broadcast
290and
291.Fn sleepq_signal
292functions.
293The
294.Fn sleepq_signal
295function awakens the highest priority thread sleeping on a wait channel while
296.Fn sleepq_broadcast
297awakens all of the threads sleeping on a wait channel.
298The
299.Fa wchan
300argument specifics which wait channel to awaken.
301The
302.Fa flags
303argument must match the sleep queue type contained in the
304.Fa flags
305argument passed to
306.Fn sleepq_add
307by the threads sleeping on the wait channel.
308If the
309.Fa pri
310argument does not equal \-1,
311then each thread that is awakened will have its priority raised to
312.Fa pri
313if it has a lower priority.
314The sleep queue chain associated with argument
315.Fa wchan
316must be locked by a prior call to
317.Fn sleepq_lock
318before calling any of these functions.
319The
320.Fa queue
321argument specifies the sub-queue, from which threads need to be woken up.
322.Pp
323A thread in an interruptible sleep can be interrupted by another thread via
324the
325.Fn sleepq_abort
326function.
327The
328.Fa td
329argument specifies the thread to interrupt.
330An individual thread can also be awakened from sleeping on a specific wait
331channel via the
332.Fn sleepq_remove
333function.
334The
335.Fa td
336argument specifies the thread to awaken and the
337.Fa wchan
338argument specifies the wait channel to awaken it from.
339If the thread
340.Fa td
341is not blocked on the the wait channel
342.Fa wchan
343then this function will not do anything,
344even if the thread is asleep on a different wait channel.
345This function should only be used if one of the other functions above is not
346sufficient.
347One possible use is waking up a specific thread from a widely shared sleep
348channel.
349.Pp
350The sleep queue interface is currently used to implement the
351.Xr sleep 9
352and
353.Xr condvar 9
354interfaces.
355Almost all other code in the kernel should use one of those interfaces rather
356than manipulating sleep queues directly.
357.Sh SEE ALSO
358.Xr condvar 9 ,
359.Xr runqueue 9 ,
360.Xr scheduler 9 ,
361.Xr sleep 9
362