CONTENTS

SYNOPSIS

DESCRIPTION

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 mutex is specified via the lock argument, and if the kernel was compiled with
.Cd "options INVARIANTS" , then the sleep queue code will perform extra checks to ensure that the mutex 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.

There are currently two types of sleep queues:

There is currently only one optional flag:

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.

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.