Re: linux sleep implementation

Ask wrote:

1> sleeping in the kernel is like this (source taken from Robert Love
Book, page 53)

add_wait_queue(q, &wait);
while (!condition) {
set_current_state(TASK_INTERRUPTIBLE);
if (signal_pending(current))
...............................

schedule();
}
set_current_state(TASK_RUNNING);
remove_wait_queue(q, &wait);

Isn't it possible that just before the schedule is invoked, the task is
pre-empted. What if it is preempted because of the event that it
intends to wait for occurs. Will it give weird result in that case ?

It can be preempted. The effect is the same as if schedule was called.
When the signal is delivered, the task changes back to TASK_RUNNING.

I vaguely remember the above code to be a workaround to avoid a race
condition when using a 2.4.x kernel. Newer kernels have a call which
allows you to go to sleep and check your condition atomically.


2> What happens after the task in TASK_INTERRUPTIBLE state receives a
signal and finshes with the associated handler. Does it go back to
sleep again ?


No, it stays in TASK_RUNNING.

Kind regards,

Iwo

.

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