linux-kernel - memory barrier question

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Message-Id: <E1Ovt6F-0005CS-RU@pomaz-ex.szeredi.hu>
Date:	Wed, 15 Sep 2010 16:36:39 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	dhowells@...hat.com
CC:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: memory barrier question

Hi,

I'm trying to understand memory barriers but not quite succeeding.

Consider the following example:

Start:
	p = NULL;
	x = 0;

CPU1:
	atomic_inc(&x);
	p = &x;

CPU2:
	if (p)
		z = atomic_read(p);

Is it possible to end up with z == 0?  What if there's a lock/unlock
before setting "p"?  What if there's a write barrier before setting
"p"?

Thanks,
Miklos
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