From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: dhowells@redhat.com, linux-kernel@vger.kernel.org,
linux-arch@vger.kernel.org
Subject: Re: memory barrier question
Date: Wed, 15 Sep 2010 21:12:42 +0200 [thread overview]
Message-ID: <201009152112.42925.rjw@sisk.pl> (raw)
In-Reply-To: <E1Ovt6F-0005CS-RU@pomaz-ex.szeredi.hu>
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
next prev parent reply other threads:[~2010-09-15 19:14 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-09-15 14:36 Miklos Szeredi
2010-09-15 19:12 ` Rafael J. Wysocki [this message]
2010-09-16 11:55 ` David Howells
2010-09-16 13:42 ` Miklos Szeredi
2010-09-16 14:30 ` David Howells
2010-09-16 15:03 ` Paul E. McKenney
2010-09-16 16:06 ` Miklos Szeredi
2010-09-16 16:37 ` Paul E. McKenney
2010-09-16 16:56 ` Miklos Szeredi
2010-09-16 17:09 ` James Bottomley
2010-09-16 17:17 ` Miklos Szeredi
2010-09-16 17:40 ` James Bottomley
2010-09-17 21:49 ` Benjamin Herrenschmidt
2010-09-17 23:12 ` Paul E. McKenney
2010-09-19 2:47 ` Benjamin Herrenschmidt
2010-09-19 15:26 ` Paul E. McKenney
2010-09-19 20:15 ` Miklos Szeredi
2010-09-19 21:59 ` Paul E. McKenney
2010-09-20 0:58 ` James Bottomley
2010-09-20 1:29 ` Paul E. McKenney
2010-09-20 16:01 ` Miklos Szeredi
2010-09-20 18:25 ` Paul E. McKenney
2010-09-20 18:57 ` Paul E. McKenney
2010-09-20 20:26 ` Michael Cree
2010-09-20 20:40 ` Paul E. McKenney
2010-09-21 14:59 ` Paul E. McKenney
2010-09-22 18:41 ` Paul E. McKenney
2010-09-18 1:12 ` Alan Cox
2010-09-16 16:50 ` Jamie Lokier
2010-09-16 16:18 ` Peter Zijlstra
2010-09-16 17:59 ` David Howells
2010-09-20 10:34 George Spelvin
2010-10-29 13:23 Memory " Tetsuo Handa
2010-10-29 15:09 ` Paul E. McKenney
2010-10-30 5:48 ` Tetsuo Handa
2010-10-30 6:11 ` Eric Dumazet
2010-10-30 12:40 ` Tetsuo Handa
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