Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751679AbaB1MNg (ORCPT ); Fri, 28 Feb 2014 07:13:36 -0500 Received: from cam-admin0.cambridge.arm.com ([217.140.96.50]:48635 "EHLO cam-admin0.cambridge.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751296AbaB1MNf (ORCPT ); Fri, 28 Feb 2014 07:13:35 -0500 Date: Fri, 28 Feb 2014 12:13:15 +0000 From: Will Deacon To: Davidlohr Bueso Cc: "linux-kernel@vger.kernel.org" , "rkuo@codeaurora.org" , "arnd@arndb.de" , "paulus@samba.org" Subject: Re: [PATCH 1/2] asm-generic: rwsem: ensure sem->cnt is only accessed via atomic_long_* Message-ID: <20140228121314.GC674@mudshark.cambridge.arm.com> References: <1393003347-24958-1-git-send-email-will.deacon@arm.com> <1393478904.3884.6.camel@buesod1.americas.hpqcorp.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1393478904.3884.6.camel@buesod1.americas.hpqcorp.net> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 27, 2014 at 05:28:24AM +0000, Davidlohr Bueso wrote: > On Fri, 2014-02-21 at 17:22 +0000, Will Deacon wrote: > > The asm-generic rwsem implementation directly acceses sem->cnt when > > performing a __down_read_trylock operation. Whilst this is probably safe > > on all architectures, we should stick to the atomic_long_* API and use > > atomic_long_read instead. > > > > Signed-off-by: Will Deacon > > --- > > include/asm-generic/rwsem.h | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h > > index bb1e2cdeb9bf..75af612f54f8 100644 > > --- a/include/asm-generic/rwsem.h > > +++ b/include/asm-generic/rwsem.h > > @@ -41,7 +41,7 @@ static inline int __down_read_trylock(struct rw_semaphore *sem) > > { > > long tmp; > > > > - while ((tmp = sem->count) >= 0) { > > + while ((tmp = atomic_long_read((atomic_long_t *)&sem->count)) >= 0) { > > That's pretty ugly, how about having infinite look and just do the tmp > assign separately from the conditional? > > It also looks like a cpu_relax() could help here between iterations. > Other than that: > > Reviewed-by: Davidlohr Bueso Actually, we should make that cmpxchg an atomic_long_cmpxchg, so the extra diff ends up looking like below. It's ugly adding a cpu_relax(), since you only want it when the cmpxchg fails (and we don't have such logic in the asm-generic __atomic_add_unless, for example). Will --->8 diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h index 603a0a11e592..2b6401f9e428 100644 --- a/include/asm-generic/rwsem.h +++ b/include/asm-generic/rwsem.h @@ -40,14 +40,16 @@ static inline void __down_read(struct rw_semaphore *sem) static inline int __down_read_trylock(struct rw_semaphore *sem) { long tmp; + atomic_long_t *cnt = (atomic_long_t *)&sem->count; - while ((tmp = atomic_long_read((atomic_long_t *)&sem->count)) >= 0) { - if (tmp == cmpxchg(&sem->count, tmp, - tmp + RWSEM_ACTIVE_READ_BIAS)) { - return 1; - } - } - return 0; + do { + tmp = atomic_long_read(cnt); + if (tmp < 0) + return 0; + } while (tmp != atomic_long_cmpxchg(cnt, tmp, + tmp + RWSEM_ACTIVE_READ_BIAS)); + + return 1; } /* -- 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/