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Date: Fri, 18 Sep 2015 15:49:37 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>, Will Deacon <will.deacon@arm.com>,
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        Al Viro <viro@zeniv.linux.org.uk>,
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        Hans Boehm <hboehm@google.com>
Subject: Re: [PATCH] kernel: fix data race in put_pid
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On Fri, Sep 18, 2015 at 03:44:53PM +0200, Oleg Nesterov wrote:
> On 09/18, Peter Zijlstra wrote:
> > +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...

Hehe, think of then as a load-store barrier; due to the 'impossibility'
of speculative stores (we'd see all kinds of random crap if you could
speculate stores).

> 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?

Nope, because its (again) a load-load order you have there.