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NAME

tsleep, msleep, wakeup — process context sleep and wakeup

SYNOPSIS

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

int
tsleep(void *ident, int priority, const char *wmesg, int timo);

int
msleep(void *ident, struct mutex *mtx, int priority, const char *wmesg, int timo);

void
wakeup(void *ident);

DESCRIPTION

These functions implement voluntary context switching. tsleep() and msleep() are used throughout the kernel whenever processing in the current context cannot 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 tsleep() 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.

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.

The msleep() function behaves just like tsleep(), but takes an additional argument:

mtx

A mutex that will be unlocked when the process is safely on the sleep queue. The mutex will be relocked at the end of msleep unless the PNORELOCK flag is set in the priority argument.

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 lock, 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.

RETURN VALUES

tsleep() and msleep() return 0 if they return as a result of a wakeup(). If they return as a result of a signal, the return value is ERESTART if the signal has the SA_RESTART property (see sigaction(2)), and EINTR otherwise. If they return as a result of a timeout, the return value is EWOULDBLOCK.

CODE REFERENCES

These functions are implemented in the file sys/kern/kern_synch.c.

SEE ALSO

hz(9), mi_switch(9), timeout(9)