linux-kernel - Re: [PATCH 02/11] wait: Provide infrastructure to deal with nested blocking

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Message-ID: <20140929210221.GA12112@redhat.com>
Date:	Mon, 29 Sep 2014 23:02:21 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, torvalds@...ux-foundation.org,
	tglx@...utronix.de, ilya.dryomov@...tank.com,
	umgwanakikbuti@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/11] wait: Provide infrastructure to deal with nested
	blocking

On 09/24, Peter Zijlstra wrote:
>
> There are a few places that call blocking primitives from wait loops,
> provide infrastructure to support this without the typical
> task_struct::state collision.
>
> We record the wakeup in wait_queue_t::flags which leaves
> task_struct::state free to be used by others.

Sorry for delay. FWIW,

Reviewed-by: Oleg Nesterov <oleg@...hat.com>

> +/*
> + * DEFINE_WAIT_FUNC(wait, woken_wait_func);
                             ^^^^^^^^^^^^^^^^
woken_wake_function ;)

> +int woken_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key)
> +{
> +	/*
> +	 * Although this function is called under waitqueue lock, LOCK
> +	 * doesn't imply write barrier and the users expects write
> +	 * barrier semantics on wakeup functions.  The following
> +	 * smp_wmb() is equivalent to smp_wmb() in try_to_wake_up()
> +	 * and is paired with set_mb() in wait_woken().
> +	 */
> +	smp_wmb(); /* C */
> +	wait->flags |= WQ_FLAG_WOKEN;

Perhaps it is just me, but I was a bit confused by the comment above wmb().
Afaics, it is not that "users expects write barrier semantics", just we
need to ensure that

	CONDITION = true;
	wait->flags |= WQ_FLAG_WOKEN;

can't be reordered (and this differs from smp_wmb() in try_to_wake_up()).
Otherwise we can obviously race with

	// wait_woken() -> set_mb()
	wait->flags &= ~WQ_FLAG_WOKEN;
	mb();

	if (CONDITION)
		break;

Oleg.

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