Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754662Ab3CLAAD (ORCPT ); Mon, 11 Mar 2013 20:00:03 -0400 Received: from mail-lb0-f170.google.com ([209.85.217.170]:58236 "EHLO mail-lb0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754484Ab3CLAAB (ORCPT ); Mon, 11 Mar 2013 20:00:01 -0400 MIME-Version: 1.0 In-Reply-To: <1362843501-31159-1-git-send-email-tom.leiming@gmail.com> References: <1362843501-31159-1-git-send-email-tom.leiming@gmail.com> Date: Tue, 12 Mar 2013 00:59:59 +0100 Message-ID: Subject: Re: [PATCH] atomic: improve atomic_inc_unless_negative/atomic_dec_unless_positive From: Frederic Weisbecker To: Ming Lei Cc: Andrew Morton , linux-kernel@vger.kernel.org, Shaohua Li , Al Viro , Paul McKenney Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2243 Lines: 56 2013/3/9 Ming Lei : > Generally, both atomic_inc_unless_negative() and > atomic_dec_unless_positive() need at least two atomic_cmpxchg() > to complete the atomic operation. In fact, the 1st atomic_cmpxchg() > is just used to read current value of the atomic variable at most times. > > Considered memory barrier, bus lock, cache walking, etc. things may be > involved in atomic_cmpxchg(), it is much expensive than atomic_read(), > which is just the simple below: > > (*(volatile int *)&(v)->counter) > > so this patch can save one extra atomic_cmpxchg() for the two > helpers under general situation, and should improve them a bit. > > Cc: Andrew Morton > Cc: Shaohua Li > Cc: Al Viro > Signed-off-by: Ming Lei > --- > include/linux/atomic.h | 28 ++++++++++++++++++---------- > 1 file changed, 18 insertions(+), 10 deletions(-) > > diff --git a/include/linux/atomic.h b/include/linux/atomic.h > index 5b08a85..aa951d8 100644 > --- a/include/linux/atomic.h > +++ b/include/linux/atomic.h > @@ -63,26 +63,34 @@ static inline int atomic_inc_not_zero_hint(atomic_t *v, int hint) > #ifndef atomic_inc_unless_negative > static inline int atomic_inc_unless_negative(atomic_t *p) > { > - int v, v1; > - for (v = 0; v >= 0; v = v1) { > - v1 = atomic_cmpxchg(p, v, v + 1); > - if (likely(v1 == v)) > + int v, t; > + > + v = atomic_read(p); > + while (1) { > + if (unlikely(v < 0)) > + return 0; But atomic_read() lacks the full memory barrier that is needed for proper atomicity here. For example if the initial value of p is -1 and another CPU just did an atomic_inc() that resulted in the new value to be 0, the above atomic_read() might return -1 because there is no guarantee it's seeing the recent update on the remote CPU. atomic_cmpxchg() OTOH provides that guarantee. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/