linux-kernel - Re: blk-throttle: Correct the placement of smp

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Date:	Wed, 8 Dec 2010 23:06:40 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: blk-throttle: Correct the placement of smp_rmb()

On 12/08, Oleg Nesterov wrote:
>
> Unfortunately, I can't prove this. You can ask
> Paul McKenney if you want the authoritative answer.

Well. I think we should ask ;) This is interesting.

Paul could you please shed a light?

Suppose we have 2 variables, A = 0 and B = 0.

	CPU0 does:

		A = 1;
		wmb();
		B = 1;

	CPU1 does:

		B = 0;
		mb();
		if (A)
			A = 2;

My understanding is: after that we can safely assume that

	B == 1 || A == 2

IOW. Either CPU1 notices that A was changed, or CPU0 "wins"
and sets B = 1 "after" CPU1. But, it is not possible that
CPU1 clears B "after" it was set by CPU0 _and_ sees A == 0.

Is it true? I think it should be true, but can't prove. This
reminds me the old (and long) discussion about STORE-MB-LOAD.
Iirc, finally it was decided that

	CPU0:				CPU1:

	A = 1;				B = 1;
	mb();				mb();
	if (B)				if (A)
		printf("Yes");			printf("Yes");

should print "Yes" at least once. This looks very similar to
the the previous example.

Thanks,

Oleg.

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