linux-kernel - Re: memory barrier question

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Message-Id: <201009152112.42925.rjw@sisk.pl>
Date:	Wed, 15 Sep 2010 21:12:42 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	dhowells@...hat.com, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org
Subject: Re: memory barrier question

On Wednesday, September 15, 2010, Miklos Szeredi wrote:
> 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?

Yes, it is.  CPU1 can reorder the two instructions in theory and if
the atomic_read() on CPU2 happens between them, it may return 0 in theory.

> What if there's a lock/unlock before setting "p"?  What if there's a write
> barrier before setting "p"?

A write barrier should help and locking functions are implicit memory barriers.

Thanks,
Rafael
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