linux-kernel - Re: [REPOST-v2] sched: Prevent wakeup to enter critical section needlessly

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Date:	Wed, 21 Nov 2012 14:03:41 +0100
From:	Ivo Sieben <meltedpianoman@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <peterz@...radead.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-v2] sched: Prevent wakeup to enter critical section needlessly

Hi

2012/11/19 Oleg Nesterov <oleg@...hat.com>:
>
> Because on a second thought I suspect this change is wrong.
>
> Just for example, please look at kauditd_thread(). It does
>
>         set_current_state(TASK_INTERRUPTIBLE);
>
>         add_wait_queue(&kauditd_wait, &wait);
>
>         if (!CONDITION)         // <-- LOAD
>                 schedule();
>
> And the last LOAD can leak into the critical section protected by
> wait_queue_head_t->lock, and it can be reordered with list_add()
> inside this critical section. In this case we can race with wake_up()
> unless it takes the same lock.
>
> Oleg.
>

I agree that I should solve my problem using the waitqueue_active()
function locally. I'll abandon this patch and fix it in the
tty_ldisc.c.

But we try to understand your fault scenario: How can the LOAD leak
into the critical section? As far as we understand the spin_unlock()
function also contains a memory barrier to prevent such a reordering
from happening.

Regards,
Ivo
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