From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752468Ab1KBUb4 (ORCPT ); Wed, 2 Nov 2011 16:31:56 -0400 Received: from e7.ny.us.ibm.com ([32.97.182.137]:34331 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752383Ab1KBUbx (ORCPT ); Wed, 2 Nov 2011 16:31:53 -0400 From: "Paul E. McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, patches@linaro.org, "Paul E. McKenney" , "Paul E. McKenney" Subject: [PATCH RFC tip/core/rcu 10/28] rcu: Disable preemption in rcu_is_cpu_idle() Date: Wed, 2 Nov 2011 13:30:31 -0700 Message-Id: <1320265849-5744-10-git-send-email-paulmck@linux.vnet.ibm.com> X-Mailer: git-send-email 1.7.3.2 In-Reply-To: <20111102203017.GA3830@linux.vnet.ibm.com> References: <20111102203017.GA3830@linux.vnet.ibm.com> x-cbid: 11110220-5806-0000-0000-000000F80A38 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Paul E. McKenney Because rcu_is_cpu_idle() is to be used to check for extended quiescent states in RCU-preempt read-side critical sections, it cannot assume that preemption is disabled. And preemption must be disabled when accessing the dyntick-idle state, because otherwise the following sequence of events could occur: 1. Task A on CPU 1 enters rcu_is_cpu_idle() and picks up the pointer to CPU 1's per-CPU variables. 2. Task B preempts Task A and starts running on CPU 1. 3. Task A migrates to CPU 2. 4. Task B blocks, leaving CPU 1 idle. 5. Task A continues execution on CPU 2, accessing CPU 1's dyntick-idle information using the pointer fetched in step 1 above, and finds that CPU 1 is idle. 6. Task A therefore incorrectly concludes that it is executing in an extended quiescent state, possibly issuing a spurious splat. Therefore, this commit disables preemption within the rcu_is_cpu_idle() function. Signed-off-by: Paul E. McKenney --- kernel/rcutree.c | 10 +++++++--- 1 files changed, 7 insertions(+), 3 deletions(-) diff --git a/kernel/rcutree.c b/kernel/rcutree.c index abb5167..c097394 100644 --- a/kernel/rcutree.c +++ b/kernel/rcutree.c @@ -556,12 +556,16 @@ void rcu_nmi_exit(void) * rcu_is_cpu_idle - see if RCU thinks that the current CPU is idle * * If the current CPU is in its idle loop and is neither in an interrupt - * or NMI handler, return true. The caller must have at least disabled - * preemption. + * or NMI handler, return true. */ int rcu_is_cpu_idle(void) { - return (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0; + int ret; + + preempt_disable(); + ret = (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0; + preempt_enable(); + return ret; } #endif /* #ifdef CONFIG_PROVE_RCU */ -- 1.7.3.2