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Date: Fri, 18 Sep 2015 15:44:53 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dmitry Vyukov <dvyukov@google.com>, Will Deacon <will.deacon@arm.com>,
        "ebiederm@xmission.com" <ebiederm@xmission.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        "mhocko@suse.cz" <mhocko@suse.cz>, LKML <linux-kernel@vger.kernel.org>,
        "ktsan@googlegroups.com" <ktsan@googlegroups.com>,
        Kostya Serebryany <kcc@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Alexander Potapenko <glider@google.com>,
        Hans Boehm <hboehm@google.com>
Subject: Re: [PATCH] kernel: fix data race in put_pid
Message-ID: <20150918134453.GA11630@redhat.com>
References: <CACT4Y+YKmhz-+sugKt86CF2715YtoVz4ee+sRfDp2Wu==A4zPQ@mail.gmail.com> <20150917174456.GA30178@redhat.com> <CACT4Y+bnqMGjMV6FHHJWsx0YaxWmRqAM+YY7b5wz-7JmFBnwjA@mail.gmail.com> <20150917180919.GA32116@redhat.com> <20150918085156.GS3816@twins.programming.kicks-ass.net> <CACT4Y+a_cBWk9E04EbsgGp6XPr-bQ=Wnt2qMgwiJJq0LSOQ02w@mail.gmail.com> <20150918092820.GA27377@arm.com> <20150918112252.GT3816@twins.programming.kicks-ass.net> <CACT4Y+YgypNXRpwO5OwvDrA9hycmFV64h5aPQ4wn66WqQC5BeA@mail.gmail.com> <20150918115637.GM3604@twins.programming.kicks-ass.net>
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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.