freebsd-skq/share/man/man9/sleepqueue.9

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.Dd March 10, 2004
.Dt SLEEPQUEUE 9
.Os
.Sh NAME
.Nm init_sleepqueues ,
.Nm sleepq_abort ,
.Nm sleepq_add ,
.Nm sleepq_alloc ,
.Nm sleepq_broadcast ,
.Nm sleepq_calc_signal_retval ,
.Nm sleepq_catch_signals ,
.Nm sleepq_free ,
.Nm sleepq_lookup ,
.Nm sleepq_release ,
.Nm sleepq_remove ,
.Nm sleepq_signal ,
.Nm sleepq_set_timeout ,
.Nm sleepq_timedwait ,
.Nm sleepq_timedwait_sig ,
.Nm sleepq_wait ,
.Nm sleepq_wait_sig
.Nd manage the queues of sleeping threads
.Sh SYNOPSIS
.In sys/param.h
.In sys/sleepqueue.h
.Ft void
.Fn init_sleepqueues "void"
.Ft void
.Fn sleepq_abort "struct thread *td"
.Ft void
.Fn sleepq_add "struct sleepqueue *sq" "void *wchan" "struct mtx *lock" "const char *wmesg" "int flags"
.Ft struct sleepqueue *
.Fn sleepq_alloc "void"
.Ft void
.Fn sleepq_broadcast "void *wchan" "int flags" "int pri"
.Ft int
.Fn sleepq_calc_signal_retval "int sig"
.Ft int
.Fn sleepq_catch_signals "void *wchan"
.Ft void
.Fn sleepq_free "struct sleepqueue *sq"
.Ft struct sleepqueue *
.Fn sleepq_lookup "void *wchan"
.Ft void
.Fn sleepq_release "void *wchan"
.Ft void
.Fn sleepq_remove "struct thread *td" "void *wchan"
.Ft void
.Fn sleepq_signal "void *wchan" "int flags" "int pri"
.Ft void
.Fn sleepq_set_timeout "void *wchan" "int timo"
.Ft int
.Fn sleepq_timedwait "void *wchan"
.Ft int
.Fn sleepq_timedwait_sig "void *wchan" "int signal_caught"
.Ft void
.Fn sleepq_wait "void *wchan"
.Ft int
.Fn sleepq_wait_sig "void *wchan"
.Sh 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.
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
.Fn sleepq_alloc
allocates an inactive sleep queue and is used to assign a sleep queue to a
thread during thread creation.
The
.Fn sleepq_free
function frees the resources associated with an inactive sleep queue and is
used to free a queue during thread destruction.
.Pp
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
.Fn init_sleepqueues
function initializes the hash table of sleep queue chains.
.Pp
The
.Fn sleepq_lookup
function locks the sleep queue chain associated with
.Fa wchan
and returns a pointer to the current active sleep queue for that wait channel
or
.Dv NULL
if there currently is not an active sleep queue.
The
.Fn sleepq_release
function unlocks the sleep queue chain associated with
.Fn wchan
and is primarily useful when aborting a pending sleep request before one of
the wait functions is called.
.Pp
The
.Fn sleepq_add
function places the current thread on the sleep queue associated with the
wait channel
.Fa wchan .
The associated sleep queue chain must be locked by a call to
.Fn sleepq_lookup
when this function is called and its return value should be passed as the
.Fa sq
parameter.
If a mutex is specified via the
.Fa lock
argument, then the sleep queue code will perform extra checks to ensure that
the mutex is held for all threads sleeping on
.Fa wchan .
The
.Fa wmesg
parameter should be a short description of
.Fa wchan .
The
.Fa flags
parameter is a bitmask consisting of the type of sleep queue being slept on
and zero or more optional flags.
There are currently two types of sleep queues:
.Bl -tag -width SLEEPQ_CONDVAR
.It Dv SLEEPQ_CONDVAR
A sleep queue used to implement condition variables.
.It Dv SLEEPQ_MSLEEP
A sleep queue used to implement
.Xr msleep 9 ,
.Xr wakeup 9
and
.Xr wakeup_one 9 .
.El
.Pp
There is currently only one optional flag:
.Bl -tag -width SLEEPQ_INTERRUPTIBLE
.It Dv SLEEPQ_INTERRUPTIBLE
The current thread is entering an interruptible sleep.
.El
.Pp
A timeout on the sleep may be specified by calling
.Fn sleepq_set_timeout
after
.Fn sleepq_add .
The
.Fa wchan
parameter should be the same value from the preceding call to
.Fn sleepq_add .
The
.Fa timo
parameter should specify the timeout value in ticks.
The thread may be marked interruptible by calling
.Fn sleepq_catch_signals
with
.Fa wchan
set to the wait channel.
The function returns a signal number if there are any pending signals for
the current thread and 0 if there is not a pending signal.
.Pp
Once the thread is ready to suspend,
one of the wait functions is called to put the thread to sleep until it is
awakened and context switch to another thread.
The
.Fn sleepq_wait
function is used for non-interruptible sleeps that do not have a timeout.
The
.Fn sleepq_timedwait
function is used for non-interruptible sleeps that have had a timeout set via
.Fn sleepq_set_timeout .
The
.Fn sleepq_wait_sig
function is used for interruptible sleeps that do not have a timeout.
The
.Fn sleepq_timedwait_sig
function is used for interruptible sleeps that do have a timeout set.
The
.Fa wchan
argument to all of the wait functions is the wait channel being slept on.
The
.Fa signal_caught
parameter to
.Fn sleepq_timedwait_sig
specifies if a previous call to
.Fn sleepq_catch_signals
found a pending signal.
.Pp
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
.Er 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
.Fn sleepq_calc_signal_retval
should be called to determine if the sleep was interrupted by a signal.
If so,
.Fn sleepq_calc_signal_retval
returns
.Er ERESTART
if the interrupting signal is restartable and
.Er EINTR
otherwise.
If the sleep was not interrupted by a signal,
.Fn sleepq_calc_signal_retval
will return 0.
.Pp
A sleeping thread is normally resumed by the
.Fn sleepq_broadcast
and
.Fn sleepq_signal
functions.
The
.Fn sleepq_signal
function awakens the highest priority thread sleeping on a wait channel while
.Fn sleepq_broadcast
awakens all of the threads sleeping on a wait channel.
The
.Fa wchan
argument specifics which wait channel to awaken.
The
.Fa flags
argument must match the sleep queue type contained in the
.Fa flags
argument passed to
.Fn sleepq_add
by the threads sleeping on the wait channel.
If the
.Fa pri
argument does not equal \-1,
then each thread that is awakened will have its priority raised to
.Fa pri
if it has a lower priority.
.Pp
A thread in an interruptible sleep can be interrupted by another thread via
the
.Fn sleepq_abort
function.
The
.Fa td
argument specifies the thread to interrupt.
An individual thread can also be awakened from sleeping on a specific wait
channel via the
.Fn sleepq_remove
function.
The
.Fa td
argument specifies the thread to awaken and the
.Fa wchan
argument specifies the wait channel to awaken it from.
If the thread
.Fa td
is not blocked on the the wait channel
.Fa 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.
.Pp
The sleep queue interface is currently used to implement the
.Xr msleep 9
and
.Xr condvar 9
interfaces.
Almost all other code in the kernel should use one of those interfaces rather
than manipulating sleep queues directly.
.Sh SEE ALSO
.Xr condvar 9 ,
.Xr msleep 9 ,
.Xr runqueue 9 ,
.Xr scheduler 9