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SLEEPQUEUE(9)	       FreeBSD Kernel Developer's Manual	 SLEEPQUEUE(9)

     init_sleepqueues, sleepq_abort, sleepq_add, sleepq_alloc,
     sleepq_broadcast, sleepq_free, sleepq_lock, sleepq_lookup,
     sleepq_release, sleepq_remove, sleepq_signal, sleepq_set_timeout,
     sleepq_set_timeout_sbt, sleepq_sleepcnt, sleepq_timedwait,
     sleepq_timedwait_sig, sleepq_type,	sleepq_wait, sleepq_wait_sig --	manage
     the queues	of sleeping threads

     #include <sys/param.h>
     #include <sys/sleepqueue.h>


     sleepq_abort(struct thread	*td);

     sleepq_add(const void *wchan, struct lock_object *lock,
	 const char *wmesg, int	flags, int queue);

     struct sleepqueue *

     sleepq_broadcast(const void *wchan, int flags, int	pri, int queue);

     sleepq_free(struct	sleepqueue *sq);

     struct sleepqueue *
     sleepq_lookup(const void *wchan);

     sleepq_lock(const void *wchan);

     sleepq_release(const void *wchan);

     sleepq_remove(struct thread *td, const void *wchan);

     sleepq_signal(const void *wchan, int flags, int pri, int queue);

     sleepq_set_timeout(const void *wchan, int timo);

     sleepq_set_timeout_sbt(const void *wchan, sbintime_t sbt, sbintime_t pr,
	 int flags);

     sleepq_sleepcnt(const void	*wchan,	int queue);

     sleepq_timedwait(const void *wchan, int pri);

     sleepq_timedwait_sig(const	void *wchan, int pri);

     sleepq_type(const void *wchan);

     sleepq_wait(const void *wchan, int	pri);

     sleepq_wait_sig(const void	*wchan,	int pri);

     Sleep queues provide a mechanism for suspending execution of a thread un-
     til some condition	is met.	 Each queue is associated with a specific wait
     channel when it is	active,	and only one queue may be associated with a
     wait channel at any given point in	time.  The implementation of each wait
     channel splits its	sleepqueue into	2 sub-queues in	order to enable	some
     optimizations on threads' wakeups.	 An active queue holds a list of
     threads that are blocked on the associated	wait channel.  Threads that
     are not blocked on	a wait channel have an associated inactive sleep
     queue.  When a thread blocks on a wait channel it donates its inactive
     sleep queue to the	wait channel.  When a thread is	resumed, the wait
     channel that it was blocked on gives it an	inactive sleep queue for later

     The sleepq_alloc()	function allocates an inactive sleep queue and is used
     to	assign a sleep queue to	a thread during	thread creation.  The
     sleepq_free() function frees the resources	associated with	an inactive
     sleep queue and is	used to	free a queue during thread destruction.

     Active sleep queues are stored in a hash table hashed on the addresses
     pointed to	by wait	channels.  Each	bucket in the hash table contains a
     sleep queue chain.	 A sleep queue chain contains a	spin mutex and a list
     of	sleep queues that hash to that specific	chain.	Active sleep queues
     are protected by their chain's spin mutex.	 The init_sleepqueues()	func-
     tion initializes the hash table of	sleep queue chains.

     The sleepq_lock() function	locks the sleep	queue chain associated with
     wait channel wchan.

     The sleepq_lookup() returns a pointer to the currently active sleep queue
     for that wait channel associated with wchan or NULL if there is no	active
     sleep queue associated with argument wchan.  It requires the sleep	queue
     chain associated with wchan to have been locked by	a prior	call to

     The sleepq_release() function unlocks the sleep queue chain associated
     with wchan() and is primarily useful when aborting	a pending sleep	re-
     quest before one of the wait functions is called.

     The sleepq_add() function places the current thread on the	sleep queue
     associated	with the wait channel wchan.  The sleep	queue chain associated
     with argument wchan must be locked	by a prior call	to sleepq_lock() when
     this function is called.  If a lock is specified via the lock argument,
     and if the	kernel was compiled with options INVARIANTS, then the sleep
     queue code	will perform extra checks to ensure that the lock is used by
     all threads sleeping on wchan.  The wmesg parameter should	be a short de-
     scription of wchan.  The flags parameter is a bitmask consisting of the
     type of sleep queue being slept on	and zero or more optional flags.  The
     queue parameter specifies the sub-queue, in which the contending thread
     will be inserted.

     There are currently three types of	sleep queues:

     SLEEPQ_CONDVAR  A sleep queue used	to implement condition variables.
     SLEEPQ_SLEEP    A sleep queue used	to implement sleep(9), wakeup(9) and
     SLEEPQ_PAUSE    A sleep queue used	to implement pause(9).

     There are currently two optional flag:

     SLEEPQ_INTERRUPTIBLE  The current thread is entering an interruptible
     SLEEPQ_STOP_ON_BDRY  When thread is entering an interruptible sleep, do
			  not stop it upon arrival of stop action, like
			  SIGSTOP.  Wake it up instead.

     A timeout on the sleep may	be specified by	calling	sleepq_set_timeout()
     after sleepq_add().  The wchan parameter should be	the same value from
     the preceding call	to sleepq_add(), and the sleep queue chain associated
     with wchan	must have been locked by a prior call to sleepq_lock().	 The
     timo parameter should specify the timeout value in	ticks.

     sleepq_set_timeout_sbt() function takes sbt argument instead of timo.  It
     allows to specify relative	or absolute wakeup time	with higher resolution
     in	form of	sbintime_t.  The parameter pr allows to	specify	wanted abso-
     lute event	precision.  The	parameter flags	allows to pass additional
     callout_reset_sbt() flags.

     Once the thread is	ready to suspend, one of the wait functions is called
     to	put the	current	thread to sleep	until it is awakened and to context
     switch to another thread.	The sleepq_wait() function is used for non-in-
     terruptible sleeps	that do	not have a timeout.  The sleepq_timedwait()
     function is used for non-interruptible sleeps that	have had a timeout set
     via sleepq_set_timeout().	The sleepq_wait_sig() function is used for in-
     terruptible sleeps	that do	not have a timeout.  The
     sleepq_timedwait_sig() function is	used for interruptible sleeps that do
     have a timeout set.  The wchan argument to	all of the wait	functions is
     the wait channel being slept on.  The sleep queue chain associated	with
     argument wchan needs to have been locked with a prior call	to
     sleepq_lock().  The pri argument is used to set the priority of the
     thread when it is awakened.  If it	is set to zero,	the thread's priority
     is	left alone.

     When the thread is	resumed, the wait functions return a non-zero value if
     the thread	was awakened due to an interrupt other than a signal or	a
     timeout.  If the sleep timed out, then EWOULDBLOCK	is returned.  If the
     sleep was interrupted by something	other than a signal, then some other
     return value will be returned.

     A sleeping	thread is normally resumed by the sleepq_broadcast() and
     sleepq_signal() functions.	 The sleepq_signal() function awakens the
     highest priority thread sleeping on a wait	channel	(if SLEEPQ_UNFAIR flag
     is	set, thread that went to sleep recently) while sleepq_broadcast()
     awakens all of the	threads	sleeping on a wait channel.  The wchan argu-
     ment specifics which wait channel to awaken.  The flags argument must
     match the sleep queue type	contained in the flags argument	passed to
     sleepq_add() by the threads sleeping on the wait channel.	If the pri ar-
     gument does not equal -1, then each thread	that is	awakened will have its
     priority raised to	pri if it has a	lower priority.	 The sleep queue chain
     associated	with argument wchan must be locked by a	prior call to
     sleepq_lock() before calling any of these functions.  The queue argument
     specifies the sub-queue, from which threads need to be woken up.

     A thread in an interruptible sleep	can be interrupted by another thread
     via the sleepq_abort() function.  The td argument specifies the thread to
     interrupt.	 An individual thread can also be awakened from	sleeping on a
     specific wait channel via the sleepq_remove() function.  The td argument
     specifies the thread to awaken and	the wchan argument specifies the wait
     channel to	awaken it from.	 If the	thread td is not blocked on the	wait
     channel wchan then	this function will not do anything, even if the	thread
     is	asleep on a different wait channel.  This function should only be used
     if	one of the other functions above is not	sufficient.  One possible use
     is	waking up a specific thread from a widely shared sleep channel.

     The sleepq_sleepcnt() function offer a simple way to retrieve the number
     of	threads	sleeping for the specified queue, given	a wchan.

     The sleepq_type() function	returns	the type of wchan associated to	a

     The sleepq_abort(), sleepq_broadcast(), and sleepq_signal() functions all
     return a boolean value.  If the return value is true, then	at least one
     thread was	resumed	that is	currently swapped out.	The caller is respon-
     sible for awakening the scheduler process so that the resumed thread will
     be	swapped	back in.  This is done by calling the kick_proc0() function
     after releasing the sleep queue chain lock	via a call to

     The sleep queue interface is currently used to implement the sleep(9) and
     condvar(9)	interfaces.  Almost all	other code in the kernel should	use
     one of those interfaces rather than manipulating sleep queues directly.

     condvar(9), runqueue(9), scheduler(9), sleep(9), timeout(9)

FreeBSD	13.0			 June 19, 2019			  FreeBSD 13.0


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