linux-kernel - Re: wait_event_interruptible

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Message-ID: <20080925164517.GA194@tv-sign.ru>
Date:	Thu, 25 Sep 2008 20:45:17 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Ani Sinha <kernel@...rban.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu
Subject: Re: wait_event_interruptible_timeout

On 09/24, Ani Sinha wrote:
>
> I have noticed an issue with wait_event_interruptible_timeout() API
> which I will try to explain below:
>
> wait_event_interruptible_timeout() is supposed to wait until one of
> the following happens:
>
> (a)    Timeout occurs (with no signals or the event of interest
> happening), in which case it returns 0
>
> (b)   The process receives a signal and wakes up prematurely (i.e.,
> before its timeout expired or the event of interest occurred). In this
> case, it returns ?ERESTARTSYS. I will come back to this later.
>
> (c)    Of course, the last obvious way to wake up is that the event of
> interest occurs and it wakes up all (or one) process on the wait
> queue. In that case, it returns the # of jiffies that is left before
> the timeout would have occurred (unslept or balance jiffies).
>
>
> What if I really wanted to sleep for timeout interval and no more?
> That is to say, if I wake up on a signal and I wanted to know how many
> jiffies I did used up while sleeping and how many I am left with (just
> like poll or select does)?

You can read jiffies before and after wait_event_interruptible_timeout().

> That way,  when I retry the API, I can tell
> it to sleep only by the amount of my balance sleeping time.

Something like

	unsigned long now = jiffies;
	ret = wait_event_interruptible_timeout(..., timeout);
	if (ret < 0) {
		next_timeout = now + timeout - jiffies;
		if (next_timeout < 0)
			next_timeout = 0;
	}

We can even add another wait_ helper which treats "timeout" as lvalue
and updates it before return. I dunno.

> Unfortunately, the interface looses this information since it
> overrides the return value with ERESTARTSYS.

Yes you are right, but

> I feel there is a need to
> modify the code in order to correct this behavior.

I'm afraid it would be a pain to modify this API. But please
do not hesitate to make the patch if you think the current API
can be improved. At worst, the patch will be nacked ;)

Oleg.

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