From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pd0-f182.google.com (mail-pd0-f182.google.com [209.85.192.182]) by kanga.kvack.org (Postfix) with ESMTP id C8B966B0031 for ; Wed, 25 Sep 2013 18:10:40 -0400 (EDT) Received: by mail-pd0-f182.google.com with SMTP id r10so256173pdi.41 for ; Wed, 25 Sep 2013 15:10:40 -0700 (PDT) Subject: [PATCH v6 1/6] rwsem: check the lock before cpmxchg in down_write_trylock From: Tim Chen In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Date: Wed, 25 Sep 2013 15:10:34 -0700 Message-ID: <1380147034.3467.63.camel@schen9-DESK> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Ingo Molnar , Andrew Morton Cc: Andrea Arcangeli , Alex Shi , Andi Kleen , Michel Lespinasse , Davidlohr Bueso , Matthew R Wilcox , Dave Hansen , Peter Zijlstra , Rik van Riel , Peter Hurley , Tim Chen , linux-kernel@vger.kernel.org, linux-mm Cmpxchg will cause the cacheline bouning when do the value checking, that cause scalability issue in a large machine (like a 80 core box). So a lock pre-read can relief this contention. Signed-off-by: Alex Shi --- include/asm-generic/rwsem.h | 8 ++++---- 1 files changed, 4 insertions(+), 4 deletions(-) diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h index bb1e2cd..5ba80e7 100644 --- a/include/asm-generic/rwsem.h +++ b/include/asm-generic/rwsem.h @@ -70,11 +70,11 @@ static inline void __down_write(struct rw_semaphore *sem) static inline int __down_write_trylock(struct rw_semaphore *sem) { - long tmp; + if (unlikely(sem->count != RWSEM_UNLOCKED_VALUE)) + return 0; - tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE, - RWSEM_ACTIVE_WRITE_BIAS); - return tmp == RWSEM_UNLOCKED_VALUE; + return cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE, + RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_UNLOCKED_VALUE; } /* -- 1.7.4.4 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org