Linux Kernel Architecture

Chapter 14: Kernel Activities

When the condition if fulfilled,finish_waitsets the task state back toTASK_RUNNINGand removes the

entry from the wait queue list.^18

In addition towait_event, the kernel defines several other functions to place the current process in a

wait queue. Their implementation is practically identical to that ofsleep_on:

<wait.h>

#define wait_event_interruptible(wq, condition)

#define wait_event_timeout(wq, condition, timeout) { ... }

#define wait_event_interruptible_timeout(wq, condition, timeout)

❑ wait_event_interruptibleuses theTASK_INTERRUPTIBLEtask state. The sleeping process can

therefore be woken up by receiving a signal.

❑ wait_event_timeoutwaits for the specified condition to be fulfilled, but stops waiting after a

time-out specified in jiffies. This prevents a process from sleeping for ever.

❑ wait_event_interruptible_timeoutputs the process to sleep so that it can be woken up by

receiving a signal. It also registers a time-out. Kernel nomenclature is usually not a place for

surprises!

Additionally the kernel defines a number of deprecated functions (sleep_on,sleep_on_timeout,

interruptible_sleep_on,andinterruptible_sleep_on_timeout) that are deprecated and not

supposed to be used in new code anymore. They still sit around for compatibility purposes.

Waking Processes

The kernel defines a series of macros that are used to wake the processes in a wait queue. They are all

based on the same function:

__wake_updelegates work to__wake_up_commonafter acquiring the necessary lock of the wait queue

head.

kernel/sched.c

static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,

int nr_exclusive, int sync, void *key)

{

wait_queue_t *curr, *next;

...