linux-kernel - Re: [REPOST] RFC: sched: Prevent wakeup to enter critical section needlessly

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Date:	Tue, 9 Oct 2012 17:17:29 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Ivo Sieben <meltedpianoman@...il.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	linux-serial@...r.kernel.org, Alan Cox <alan@...ux.intel.com>,
	Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [REPOST] RFC: sched: Prevent wakeup to enter critical section
	needlessly

On 10/09, Peter Zijlstra wrote:
>
> One thing you might need to consider is the memory ordering, will the
> list_empty -- either careful or not -- observe the right list pointer,
> or could it -- when racing with wait_event()/prepare_to_wait() --
> observe a stale value. Or.. is that all already covered in on the use
> site.

I agree.

Without spin_lock(q->lock) (or some other barriers) wait_event-like
code can miss an event.

wait_event:

	prepare_to_wait(wq)	// takes wq->lock

	if (!CONDITION)
		schedule();

Now,

	CONDITION = 1;
	wake_up(wq);

at least need the full mb() before lits_empty().

Oleg.

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