Dear Domen,
Thanks for your reply first.
>I understand it this way:
>- condition
> Just checking the condition is one way (if you don't have a wake_up
> source, like an interrupt), but that's not really what wait_event does.
> It would be something like
> while (condition) {
> msleep(10);
> }
> There was some talk on poll_wait(), but I don't know what happened to
> it.
So you mean in my senario (wake the process up in the interrupt handler), I
needn't to use wake_up at all? A "condition == true" in the interrupt
handler is enough to wake the sleeping process up? Am I right?
I checked the source code in linux/wait.h and here is the defination of
wait_event:
#define __wait_event(wq, condition)
do {
DEFINE_WAIT(__wait);
for (;;) { \
prepare_to_wait(&wq, &__wait, TASK_UNINTERRUPTIBLE);
if (condition)
break;
schedule();
}
finish_wait(&wq, &__wait);
} while (0)
#define wait_event(wq, condition)
do {
if (condition)
break;
__wait_event(wq, condition);
} while (0)
From the source code, it seems like that this mechanism doesn't use
msleep(), like what you mentioned, to release its executing. Instead, it
uses schedule() to do that.
>- wake_up
> Just wake_up isn't enough, you get a race:
> | interrupt handler | process |
> ------------------------------------------
> | do_something() | |
> | wake_up() | |
> | ... | wait on wq |
>
> And so you have a process waiting on waitqueue, that just missed the
> wakeup. Obviously should not be used.
>
>- wake_up & condition
> | interrupt handler | process |
> ------------------------------------------
> | flag = 1 | |
> | wake_up() | |
> | ... | wait_event |
> | ... | flag = 0 |
>
> This will work properly and if wait_event misses a wake_up, the
> condition check (flag) will kick in before putting it to sleep.
>
Thanks for your explaining on the race problem. I can understand this now.
However I still cannot understand, is such a problem: In the above figures
for my case, if flag=1 could wake the process up, then what's the use of
wake_up()? From my understanding if the condition turns true, then the
process which depends on this condition will be waken up. Thus what's the
exact use of wake_up()? Also in my program, I tried to remove wake_up()
sentence and it seems that there is no difference on the result.
Thanks for the explanation.
BR
Ming
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