From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.6 required=3.0 tests=DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,T_DKIM_INVALID, USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A0687ECDFB3 for ; Tue, 17 Jul 2018 19:35:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5A0CD20693 for ; Tue, 17 Jul 2018 19:35:41 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="key not found in DNS" (0-bit key) header.d=codeaurora.org header.i=@codeaurora.org header.b="BJGkK2VC"; dkim=fail reason="key not found in DNS" (0-bit key) header.d=codeaurora.org header.i=@codeaurora.org header.b="BJGkK2VC" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5A0CD20693 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=codeaurora.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730137AbeGQUJm (ORCPT ); Tue, 17 Jul 2018 16:09:42 -0400 Received: from smtp.codeaurora.org ([198.145.29.96]:52252 "EHLO smtp.codeaurora.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730000AbeGQUJm (ORCPT ); Tue, 17 Jul 2018 16:09:42 -0400 Received: by smtp.codeaurora.org (Postfix, from userid 1000) id 506746078C; Tue, 17 Jul 2018 19:35:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=codeaurora.org; s=default; t=1531856136; bh=S4LpACIPG45/tgspNNa/diPdaepLoM8z7UzFPNzhXxI=; h=From:To:Cc:Subject:Date:From; b=BJGkK2VC7UB3S7AUlPVqRuaXe3daclYhCEMW+TcJ0AmpzZfTkML4+Bu8WSlYXRh7h CEUR4bLzxxy7QaVCJewImdVcFnOS1wiH4Jxee+6q8JBiV41bBL2gLRaU4qI8BLYS8a 71e8bqpvn4HwgpYtPKg6wtNgrHHRUKL8EGcqJzWA= Received: from isaacm-linux.qualcomm.com (i-global254.qualcomm.com [199.106.103.254]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-SHA256 (128/128 bits)) (No client certificate requested) (Authenticated sender: isaacm@smtp.codeaurora.org) by smtp.codeaurora.org (Postfix) with ESMTPSA id 70A5B602BC; Tue, 17 Jul 2018 19:35:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=codeaurora.org; s=default; t=1531856136; bh=S4LpACIPG45/tgspNNa/diPdaepLoM8z7UzFPNzhXxI=; h=From:To:Cc:Subject:Date:From; b=BJGkK2VC7UB3S7AUlPVqRuaXe3daclYhCEMW+TcJ0AmpzZfTkML4+Bu8WSlYXRh7h CEUR4bLzxxy7QaVCJewImdVcFnOS1wiH4Jxee+6q8JBiV41bBL2gLRaU4qI8BLYS8a 71e8bqpvn4HwgpYtPKg6wtNgrHHRUKL8EGcqJzWA= DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org 70A5B602BC Authentication-Results: pdx-caf-mail.web.codeaurora.org; dmarc=none (p=none dis=none) header.from=codeaurora.org Authentication-Results: pdx-caf-mail.web.codeaurora.org; spf=none smtp.mailfrom=isaacm@codeaurora.org From: "Isaac J. Manjarres" To: peterz@infradead.org, matt@codeblueprint.co.uk, mingo@kernel.org, tglx@linutronix.de, bigeasy@linutronix.de Cc: "Isaac J. Manjarres" , linux-kernel@vger.kernel.org, psodagud@codeaurora.org, gregkh@linuxfoundation.org, pkondeti@codeaurora.org, stable@vger.kernel.org Subject: [PATCH] stop_machine: Disable preemption after queueing stopper threads Date: Tue, 17 Jul 2018 12:35:29 -0700 Message-Id: <1531856129-9871-1-git-send-email-isaacm@codeaurora.org> X-Mailer: git-send-email 1.9.1 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This commit: 9fb8d5dc4b64 ("stop_machine, Disable preemption when waking two stopper threads") does not fully address the race condition that can occur as follows: On one CPU, call it CPU 3, thread 1 invokes cpu_stop_queue_two_works(2, 3,...), and the execution is such that thread 1 queues the works for migration/2 and migration/3, and is preempted after releasing the locks for migration/2 and migration/3, but before waking the threads. Then, On CPU 2, a kworker, call it thread 2, is running, and it invokes cpu_stop_queue_two_works(1, 2,...), such that thread 2 queues the works for migration/1 and migration/2. Meanwhile, on CPU 3, thread 1 resumes execution, and wakes migration/2 and migration/3. This means that when CPU 2 releases the locks for migration/1 and migration/2, but before it wakes those threads, it can be preempted by migration/2. If thread 2 is preempted by migration/2, then migration/2 will execute the first work item successfully, since migration/3 was woken up by CPU 3, but when it goes to execute the second work item, it disables preemption, calls multi_cpu_stop(), and thus, CPU 2 will wait forever for migration/1, which should have been woken up by thread 2. However migration/1 cannot be woken up by thread 2, since it is a kworker, so it is affine to CPU 2, but CPU 2 is running migration/2 with preemption disabled, so thread 2 will never run. Disable preemption after queueing works for stopper threads to ensure that the operation of queueing the works and waking the stopper threads is atomic. Fixes: 9fb8d5dc4b64 ("stop_machine, Disable preemption when waking two stopper threads") Co-Developed-by: Prasad Sodagudi Co-Developed-by: Pavankumar Kondeti Signed-off-by: Isaac J. Manjarres Signed-off-by: Prasad Sodagudi Signed-off-by: Pavankumar Kondeti Cc: stable@vger.kernel.org --- kernel/stop_machine.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c index 1ff523d..e190d1e 100644 --- a/kernel/stop_machine.c +++ b/kernel/stop_machine.c @@ -260,6 +260,15 @@ static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1, err = 0; __cpu_stop_queue_work(stopper1, work1, &wakeq); __cpu_stop_queue_work(stopper2, work2, &wakeq); + /* + * The waking up of stopper threads has to happen + * in the same scheduling context as the queueing. + * Otherwise, there is a possibility of one of the + * above stoppers being woken up by another CPU, + * and preempting us. This will cause us to n ot + * wake up the other stopper forever. + */ + preempt_disable(); unlock: raw_spin_unlock(&stopper2->lock); raw_spin_unlock_irq(&stopper1->lock); @@ -271,7 +280,6 @@ static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1, } if (!err) { - preempt_disable(); wake_up_q(&wakeq); preempt_enable(); } -- The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project