From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757345Ab1FJNNW (ORCPT ); Fri, 10 Jun 2011 09:13:22 -0400 Received: from www.linutronix.de ([62.245.132.108]:37073 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757228Ab1FJNI6 (ORCPT ); Fri, 10 Jun 2011 09:08:58 -0400 Date: Fri, 10 Jun 2011 15:08:55 +0200 (CEST) From: Thomas Gleixner To: LKML cc: Peter Zijlstra , Ingo Molnar , "Paul E. McKenney" Subject: [PATCH] sched; Simplify mutex_spin_on_owner() Message-ID: User-Agent: Alpine 2.02 (LFD 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org It does not make sense to rcu_read_lock/unlock() in every loop iteration while spinning on the mutex. Move the rcu protection once outside the loop. Also simplify the return path to always check for lock->owner == NULL which meets the requirements of both owner changed and need_resched() caused loop exits. Signed-off-by: Thomas Gleixner --- kernel/sched.c | 25 +++++++++---------------- 1 file changed, 9 insertions(+), 16 deletions(-) Index: linux-2.6/kernel/sched.c =================================================================== --- linux-2.6.orig/kernel/sched.c +++ linux-2.6/kernel/sched.c @@ -4306,11 +4306,8 @@ EXPORT_SYMBOL(schedule); static inline bool owner_running(struct mutex *lock, struct task_struct *owner) { - bool ret = false; - - rcu_read_lock(); if (lock->owner != owner) - goto fail; + return false; /* * Ensure we emit the owner->on_cpu, dereference _after_ checking @@ -4320,11 +4317,7 @@ static inline bool owner_running(struct */ barrier(); - ret = owner->on_cpu; -fail: - rcu_read_unlock(); - - return ret; + return owner->on_cpu; } /* @@ -4336,21 +4329,21 @@ int mutex_spin_on_owner(struct mutex *lo if (!sched_feat(OWNER_SPIN)) return 0; + rcu_read_lock(); while (owner_running(lock, owner)) { if (need_resched()) - return 0; + break; arch_mutex_cpu_relax(); } + rcu_read_unlock(); /* - * If the owner changed to another task there is likely - * heavy contention, stop spinning. + * We break out the loop above on need_resched() and when the + * owner changed, which is a sign for heavy contention. Return + * success only when lock->owner is NULL. */ - if (lock->owner) - return 0; - - return 1; + return lock->owner == NULL; } #endif