The interfaces described in this manual page are obsolete and will be removed from a future version of the system.
Please see the condvar(9),
mutex(9),
and rwlock(9) manual pages for information on kernel synchronisation primitives.
These functions implement voluntary context switching.
ltsleep() and
tsleep() are used throughout the kernel whenever processing in the current context can not continue for any of the following reasons:
•
The current process needs to await the results of a pending I/O operation.
•
The current process needs resources (e.g., memory) which are temporarily unavailable.
•
The current process wants access to data-structures which are locked by other processes.
The function
wakeup() is used to notify sleeping processes of possible changes to the condition that caused them to go to sleep. Typically, an awakened process will -- after it has acquired a context again -- retry the action that blocked its operation to see if the “blocking” condition has cleared.
The
ltsleep() function takes the following arguments:
ident
An identifier of the “wait channel” representing the resource for which the current process needs to wait. This typically is the virtual address of some kernel data-structure related to the resource for which the process is contending. The same identifier must be used in a call to wakeup() to get the process going again. ident should not be NULL.
priority
The process priority to be used when the process is awakened and put on the queue of runnable processes. This mechanism is used to optimize “throughput” of processes executing in kernel mode. If the flag PCATCH is OR'ed into priority the process checks for posted signals before and after sleeping. If the flag PNORELOCK is OR'ed into priority, slock is NOT re-locked after process resume.
wmesg
A pointer to a character string indicating the reason a process is sleeping. The kernel does not use the string, but makes it available (through the process structure field
p_wmesg) for user level utilities such as
ps(1).
timo
If non-zero, the process will sleep for at most timo/hz seconds. If this amount of time elapses and no wakeup(ident) has occurred, and no signal (if PCATCH was set) was posted, tsleep() will return EWOULDBLOCK.
slock
If not NULL, the slock interlock is unlocked once the scheduler lock is acquired. Unless PNORELOCK was set, slock is locked again once the process is resumed from sleep. This provides wakeup-before-sleep condition protection facility.
The
tsleep() macro is functionally equivalent to:
ltsleep(ident, priority, wmesg, timo, NULL)
The
wakeup() function will mark all processes which are currently sleeping on the identifier
ident as runnable. Eventually, each of the processes will resume execution in the kernel context, causing a return from
tsleep(). Note that processes returning from sleep should always re-evaluate the conditions that blocked them, since a call to
wakeup() merely signals a
possible change to the blocking conditions. For example, when two or more processes are waiting for an exclusive-access lock (see
lock(9)), only one of them will succeed in acquiring the lock when it is released. All others will have to go back to sleep and wait for the next opportunity.