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Date: Tue, 1 Sep 2015 11:59:23 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Michal Hocko <mhocko@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        David Howells <dhowells@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jonathan Corbet <corbet@lwn.net>
Subject: Re: wake_up_process implied memory barrier clarification
Message-ID: <20150901095923.GB31368@redhat.com>
References: <20150827131444.GE27052@dhcp22.suse.cz> <20150827182654.GA12191@redhat.com> <20150828145121.GG5301@dhcp22.suse.cz> <20150828160637.GA4393@redhat.com> <20150829092514.GA3240@fixme-laptop.cn.ibm.com> <20150829142707.GA19263@redhat.com> <20150831003719.GC924@fixme-laptop.cn.ibm.com> <20150831183335.GA26333@redhat.com> <20150831203739.GX4029@linux.vnet.ibm.com> <20150901034014.GD1071@fixme-laptop.cn.ibm.com>
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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.