linux-kernel - Re: wake_up_process implied memory barrier clarification

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Date:	Tue, 1 Sep 2015 11:59:23 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Michal Hocko <mhocko@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>
Subject: Re: wake_up_process implied memory barrier clarification

On 09/01, Boqun Feng wrote:
>
> But I'm still a little confused at Oleg's words:
>
> "What is really important is that we have a barrier before we _read_ the
> task state."
>
> I read is as "What is really important is that we have a barrier before
> we _read_ the task state and _after_ we write the CONDITION", if I don't
> misunderstand Oleg, this means a STORE-barrier-LOAD sequence,

Yes, exactly.

Let's look at this trivial code again,

	CONDITION = 1;
	wake_up_process();

note that try_to_wake_up() does

	if (!(p->state & state))
		goto out;

If this LOAD could be reordered with STORE(CONDITION) above we can obviously
race with

	set_current_state(...);
	if (!CONDITION)
		schedule();

See the comment at the start of try_to_wake_up(). And again, again, please
note that initially the only documented behaviour of smp_mb__before_spinlock()
was the STORE - LOAD serialization. This is what try_to_wake_up() needs, it
doesn't actually need the write barrier after STORE(CONDITION).

And just in case, wake_up() differs in a sense that it doesn't even need
that STORE-LOAD barrier in try_to_wake_up(), we can rely on
wait_queue_head_t->lock. Assuming that wake_up() pairs with the "normal"
wait_event()-like code.

> which IIUC
> can't pair with anything.

It pairs with the barrier implied by set_current_state().

Oleg.

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