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

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Message-ID: <20150918134453.GA11630@redhat.com>
Date:	Fri, 18 Sep 2015 15:44:53 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Dmitry Vyukov <dvyukov@...gle.com>,
	Will Deacon <will.deacon@....com>,
	"ebiederm@...ssion.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" <mhocko@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>,
	"ktsan@...glegroups.com" <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

On 09/18, Peter Zijlstra wrote:
>
> Provide atomic_read_ctrl() to mirror READ_ONCE_CTRL(), such that we can
> more conveniently use atomics in control dependencies.
>
> Since we can assume atomic_read() implies a READ_ONCE(), we must only
> emit an extra smp_read_barrier_depends() in order to upgrade to
> READ_ONCE_CTRL() semantics.

...

> +static inline int atomic_read_ctrl(atomic_t *v)
> +{
> +	int val = atomic_read(v);
> +	smp_read_barrier_depends(); /* Enforce control dependency. */
> +	return val;
> +}

Help. I am starting to think that the control dependencies is even more
hard to understand that memory barriers...

So I assume that if we have

	int X = 0;
	atomic_t Y = ATOMIC_INIT(0);

	void w(void)
	{
		X = 1;
		atomic_inc_return(&Y);
	}

then

	void r(void)
	{
		if (atomic_read_ctrl(&Y))
			BUG_ON(X == 0);
	}

should be correct?  Why?

If not then I am even more confused.

Oleg.

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