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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_timedwait,
     sleepq_timedwait_sig, sleepq_wait, sleepq_wait_sig - manage the queues of
     sleeping threads

SYNOPSIS

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

     void
     init_sleepqueues(void);

     void
     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);

     void
     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);

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

     void
     sleepq_set_timeout(void *wchan, int timo);

     int
     sleepq_timedwait(void *wchan);

     int
     sleepq_timedwait_sig(void *wchan, int signal_caught);

     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()
     function 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 is currently only one optional flag:

     SLEEPQ_INTERRUPTIBLE  The current thread is entering an interruptible
                           sleep.

     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
     possible use is waking up a specific thread from a widely shared sleep
     channel.

     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 ALSO

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