From mboxrd@z Thu Jan 1 00:00:00 1970 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752559AbdBETPy (ORCPT + 2 others); Sun, 5 Feb 2017 14:15:54 -0500 Received: from wtarreau.pck.nerim.net ([62.212.114.60]:27273 "EHLO 1wt.eu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752527AbdBETPw (ORCPT ); Sun, 5 Feb 2017 14:15:52 -0500 From: Willy Tarreau To: linux-kernel@vger.kernel.org, stable@vger.kernel.org, linux@roeck-us.net Cc: Peter Zijlstra , Linus Torvalds , Mike Galbraith , Thomas Gleixner , Ingo Molnar , Willy Tarreau Subject: [PATCH 3.10 002/319] sched/core: Fix an SMP ordering race in try_to_wake_up() vs. schedule() Date: Sun, 5 Feb 2017 20:09:06 +0100 Message-Id: <1486322063-7558-3-git-send-email-w@1wt.eu> X-Mailer: git-send-email 2.8.0.rc2.1.gbe9624a In-Reply-To: <1486322063-7558-1-git-send-email-w@1wt.eu> References: <1486322063-7558-1-git-send-email-w@1wt.eu> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: From: Peter Zijlstra commit ecf7d01c229d11a44609c0067889372c91fb4f36 upstream. Oleg noticed that its possible to falsely observe p->on_cpu == 0 such that we'll prematurely continue with the wakeup and effectively run p on two CPUs at the same time. Even though the overlap is very limited; the task is in the middle of being scheduled out; it could still result in corruption of the scheduler data structures. CPU0 CPU1 set_current_state(...) context_switch(X, Y) prepare_lock_switch(Y) Y->on_cpu = 1; finish_lock_switch(X) store_release(X->on_cpu, 0); try_to_wake_up(X) LOCK(p->pi_lock); t = X->on_cpu; // 0 context_switch(Y, X) prepare_lock_switch(X) X->on_cpu = 1; finish_lock_switch(Y) store_release(Y->on_cpu, 0); schedule(); deactivate_task(X); X->on_rq = 0; if (X->on_rq) // false if (t) while (X->on_cpu) cpu_relax(); context_switch(X, ..) finish_lock_switch(X) store_release(X->on_cpu, 0); Avoid the load of X->on_cpu being hoisted over the X->on_rq load. Reported-by: Oleg Nesterov Signed-off-by: Peter Zijlstra (Intel) Cc: Linus Torvalds Cc: Mike Galbraith Cc: Peter Zijlstra Cc: Thomas Gleixner Signed-off-by: Ingo Molnar Signed-off-by: Willy Tarreau --- kernel/sched/core.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index c8afc2c..6a366f9 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1528,6 +1528,25 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) #ifdef CONFIG_SMP /* + * Ensure we load p->on_cpu _after_ p->on_rq, otherwise it would be + * possible to, falsely, observe p->on_cpu == 0. + * + * One must be running (->on_cpu == 1) in order to remove oneself + * from the runqueue. + * + * [S] ->on_cpu = 1; [L] ->on_rq + * UNLOCK rq->lock + * RMB + * LOCK rq->lock + * [S] ->on_rq = 0; [L] ->on_cpu + * + * Pairs with the full barrier implied in the UNLOCK+LOCK on rq->lock + * from the consecutive calls to schedule(); the first switching to our + * task, the second putting it to sleep. + */ + smp_rmb(); + + /* * If the owning (remote) cpu is still in the middle of schedule() with * this task as prev, wait until its done referencing the task. */ -- 2.8.0.rc2.1.gbe9624a