linux-kernel - Re: [PATCH] kernel: fix data race in put

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Message-ID: <20150918153012.GA15521@redhat.com>
Date:	Fri, 18 Sep 2015 17:30:12 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Dmitry Vyukov <dvyukov@...gle.com>, ebiederm@...ssion.com,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>, mhocko@...e.cz,
	LKML <linux-kernel@...r.kernel.org>, ktsan@...glegroups.com,
	Kostya Serebryany <kcc@...gle.com>,
	Andrey Konovalov <andreyknvl@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Hans Boehm <hboehm@...gle.com>
Subject: Re: [PATCH] kernel: fix data race in put_pid

I can't resist. You certainly don't need this spam, but

On 09/18, Peter Zijlstra wrote:
>
> On Fri, Sep 18, 2015 at 03:28:44PM +0200, Oleg Nesterov wrote:
> >
> > And again, I simply can't understand if this code
> >
> > 	if (READ_ONCE_CTRL(Y))
> > 		BUG_ON(X == 0);
> >
> > to me it does _not_ look correct in theory.
>
> So control dependencies provide a load-store barrier. Your examples
> above rely on a load-load barrier; BUG_ON(X == 0) is a load.

Yes, and this is just obvious.

Yet I was confused because reading other emails I misunderstood the
proposed semantics of atomic_read_ctrl(), so I started to suspect
that in fact _CTRL() does more than I used to think.

Nevermind, sorry for noise, thanks to all.

Oleg.

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