SLEEPQUEUE(9) BSD Kernel Developer's Manual SLEEPQUEUE(9)NAME
init_sleepqueues, sleepq_abort, sleepq_add, sleepq_alloc,
sleepq_broadcast, sleepq_calc_signal_retval, sleepq_catch_signals,
sleepq_free, sleepq_lock, sleepq_lookup, sleepq_release, sleepq_remove,
sleepq_signal, sleepq_set_timeout, sleepq_sleepcnt, sleepq_timedwait,
sleepq_timedwait_sig, sleepq_type, sleepq_wait, sleepq_wait_sig — manage
the queues of sleeping threads
SYNOPSIS
#include <sys/param.h>
#include <sys/sleepqueue.h>
void
init_sleepqueues(void);
int
sleepq_abort(struct thread *td);
void
sleepq_add(void *wchan, struct lock_object *lock, const char *wmesg,
int flags, int queue);
struct sleepqueue *
sleepq_alloc(void);
int
sleepq_broadcast(void *wchan, int flags, int pri, int queue);
int
sleepq_calc_signal_retval(int sig);
int
sleepq_catch_signals(void *wchan);
void
sleepq_free(struct sleepqueue *sq);
struct sleepqueue *
sleepq_lookup(void *wchan);
void
sleepq_lock(void *wchan);
void
sleepq_release(void *wchan);
void
sleepq_remove(struct thread *td, void *wchan);
int
sleepq_signal(void *wchan, int flags, int pri, int queue);
void
sleepq_set_timeout(void *wchan, int timo);
u_int
sleepq_sleepcnt(void *wchan, int queue);
int
sleepq_timedwait(void *wchan);
int
sleepq_timedwait_sig(void *wchan, int signal_caught);
int
sleepq_type(void *wchan);
void
sleepq_wait(void *wchan);
int
sleepq_wait_sig(void *wchan);
DESCRIPTION
Sleep queues provide a mechanism for suspending execution of a thread
until 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
use.
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
sleepq_lock().
The sleepq_release() function unlocks the sleep queue chain associated
with wchan() and is primarily useful when aborting a pending sleep
request 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
description 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
wakeup_one(9).
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
sleep.
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.
The current thread may be marked interruptible by calling
sleepq_catch_signals() with wchan set to the wait channel. This function
returns a signal number if there are any pending signals for the current
thread and 0 if there is not a pending signal. The sleep queue chain
associated with argument wchan should have been locked by a prior call to
sleepq_lock().
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-
interruptible 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
interruptible 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 signal_caught parameter to sleepq_timedwait_sig()
specifies if a previous call to sleepq_catch_signals() found a pending
signal.
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. If zero is returned after resuming from
an interruptible sleep, then sleepq_calc_signal_retval() should be called
to determine if the sleep was interrupted by a signal. If so,
sleepq_calc_signal_retval() returns ERESTART if the interrupting signal
is restartable and EINTR otherwise. If the sleep was not interrupted by
a signal, sleepq_calc_signal_retval() will return 0.
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 while
sleepq_broadcast() awakens all of the threads sleeping on a wait channel.
The wchan argument 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 argument 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 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 pos‐
sible use is waking up a specific thread from a widely shared sleep chan‐
nel.
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
sleepqueue.
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
sleepq_release().
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.
SEE ALSOcondvar(9), runqueue(9), scheduler(9), sleep(9)BSD January 25, 2010 BSD
