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Date: Sat, 10 Dec 2011 23:57:14 +0800 From: Ming Lei <tom.leiming@...il.com> To: Peter Zijlstra <a.p.zijlstra@...llo.nl> Cc: gregkh@...e.de, akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, ostrikov@...dia.com, adobriyan@...il.com, eric.dumazet@...il.com, mingo@...e.hu, Oliver Neukum <oneukum@...e.de> Subject: Re: [PATCH 3/3] kref: Remove the memory barriers On Sat, Dec 10, 2011 at 10:58 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote: > On Sat, 2011-12-10 at 22:07 +0800, Ming Lei wrote: >> > While true, it fails to show why this is a problem. I say it is not a >> >> IMO, the added two barriers are pairs of the implicit barrier in kref_put, so >> that we can order between kref_init/kref_get and kref_put. > > Yeah so? I think so, see below: CPU0 CPU1 atomic_set(v) smp_mb() smp_mb() atomic_dec_and_test(v) Without the barrier after atomic_set, CPU1 may see a stale value of v first, then decrease it, so may miss a release operation. The pair of smp_mb can make order between atomic_set and atomic_dec_and_test, can't it? > If there's a destruction race with kref_put() the barrier won't Sorry, could you say what the destruction race is? > solve it. Other than that the actual order of get/put is irrelevant. thanks, -- Ming Lei -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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