From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751769AbaHOQ3D (ORCPT ); Fri, 15 Aug 2014 12:29:03 -0400 Received: from e31.co.us.ibm.com ([32.97.110.149]:49895 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751538AbaHOQ25 (ORCPT ); Fri, 15 Aug 2014 12:28:57 -0400 From: "Paul E. McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com, "Paul E. McKenney" Subject: [PATCH v6 tip/core/rcu 14/17] rcu: Remove redundant preempt_disable() from rcu_note_voluntary_context_switch() Date: Fri, 15 Aug 2014 09:28:42 -0700 Message-Id: <1408120125-30479-14-git-send-email-paulmck@linux.vnet.ibm.com> X-Mailer: git-send-email 1.8.1.5 In-Reply-To: <1408120125-30479-1-git-send-email-paulmck@linux.vnet.ibm.com> References: <20140815162820.GA28967@linux.vnet.ibm.com> <1408120125-30479-1-git-send-email-paulmck@linux.vnet.ibm.com> X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14081516-8236-0000-0000-000004A2F588 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Paul E. McKenney" In theory, synchronize_sched() requires a read-side critical section to order against. In practice, preemption can be thought of as being disabled across every machine instruction, at least for those machine instructions that are not in the idle loop and not on offline CPUs. So this commit removes the redundant preempt_disable() from rcu_note_voluntary_context_switch(). Please note that the single instruction in question is the store of zero to ->rcu_tasks_holdout. The "if" is simply a performance optimization that avoids unnecessary stores. To see this, keep in mind that both the "if" condition and the store are in a quiescent state. Therefore, even if the task is preempted for a full grace period (presumably due to its having done a context switch beforehand), the store will be recording a legitimate quiescent state. Reported-by: Lai Jiangshan Signed-off-by: Paul E. McKenney Conflicts: include/linux/rcupdate.h --- include/linux/rcupdate.h | 2 -- 1 file changed, 2 deletions(-) diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 494b1b512c81..cc6858666b6d 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -326,10 +326,8 @@ static inline void rcu_user_hooks_switch(struct task_struct *prev, extern struct srcu_struct tasks_rcu_exit_srcu; #define rcu_note_voluntary_context_switch(t) \ do { \ - preempt_disable(); /* Exclude synchronize_sched(); */ \ if (ACCESS_ONCE((t)->rcu_tasks_holdout)) \ ACCESS_ONCE((t)->rcu_tasks_holdout) = false; \ - preempt_enable(); \ } while (0) #else /* #ifdef CONFIG_TASKS_RCU */ #define TASKS_RCU(x) do { } while (0) -- 1.8.1.5