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=-9.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 BB10CC433E1 for ; Mon, 17 Aug 2020 19:14:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9E99F2072D for ; Mon, 17 Aug 2020 19:14:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1597691666; bh=OcRL81cZfVRoE/k4wxL8Y+ucO+Gbpj6KCXAPEkW9zzU=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=QsoER0RHdYsmPSeDRaROt9tSLP56pfRm8vq7N7vEVGXPFjEy9mGLR391tlFuu++NZ wQmOfJc3N8V+B3xGo+vIVueuTDZOezO8QioZdaURTcBlN+XbA+pKH541GBPYgLvfIZ 6OC7CASgq8JamaD+Duu2z7TeOPzp5kMqB4L0G0A0= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387631AbgHQTOE (ORCPT ); Mon, 17 Aug 2020 15:14:04 -0400 Received: from mail.kernel.org ([198.145.29.99]:48964 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730340AbgHQPkQ (ORCPT ); Mon, 17 Aug 2020 11:40:16 -0400 Received: from localhost (83-86-89-107.cable.dynamic.v4.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 3F8CC23343; Mon, 17 Aug 2020 15:40:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1597678815; bh=OcRL81cZfVRoE/k4wxL8Y+ucO+Gbpj6KCXAPEkW9zzU=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=WPP6nFFwshbLWyiMKm+i453zewa/Oc2++k1+lqXEkinH4GkgaTfbuUPjG0Lbp399v 0mFRL+cpTviGgGoY6n89ID9ZYrpdRfhQ5CZHmZl1nNhJkBlz1wGmzhDak2UUiG9jmA mtM29L5oZS2TXjQS+zrtCUfMFc+tpguZ/uFWnork= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Peter Zijlstra , Jann Horn , Oleg Nesterov , Jens Axboe Subject: [PATCH 5.8 464/464] task_work: only grab task signal lock when needed Date: Mon, 17 Aug 2020 17:16:57 +0200 Message-Id: <20200817143856.004716384@linuxfoundation.org> X-Mailer: git-send-email 2.28.0 In-Reply-To: <20200817143833.737102804@linuxfoundation.org> References: <20200817143833.737102804@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Jens Axboe commit ebf0d100df0731901c16632f78d78d35f4123bc4 upstream. If JOBCTL_TASK_WORK is already set on the targeted task, then we need not go through {lock,unlock}_task_sighand() to set it again and queue a signal wakeup. This is safe as we're checking it _after_ adding the new task_work with cmpxchg(). The ordering is as follows: task_work_add() get_signal() -------------------------------------------------------------- STORE(task->task_works, new_work); STORE(task->jobctl); mb(); mb(); LOAD(task->jobctl); LOAD(task->task_works); This speeds up TWA_SIGNAL handling quite a bit, which is important now that io_uring is relying on it for all task_work deliveries. Cc: Peter Zijlstra Cc: Jann Horn Acked-by: Oleg Nesterov Signed-off-by: Jens Axboe Signed-off-by: Greg Kroah-Hartman --- kernel/signal.c | 16 +++++++++++++++- kernel/task_work.c | 8 +++++++- 2 files changed, 22 insertions(+), 2 deletions(-) --- a/kernel/signal.c +++ b/kernel/signal.c @@ -2541,7 +2541,21 @@ bool get_signal(struct ksignal *ksig) relock: spin_lock_irq(&sighand->siglock); - current->jobctl &= ~JOBCTL_TASK_WORK; + /* + * Make sure we can safely read ->jobctl() in task_work add. As Oleg + * states: + * + * It pairs with mb (implied by cmpxchg) before READ_ONCE. So we + * roughly have + * + * task_work_add: get_signal: + * STORE(task->task_works, new_work); STORE(task->jobctl); + * mb(); mb(); + * LOAD(task->jobctl); LOAD(task->task_works); + * + * and we can rely on STORE-MB-LOAD [ in task_work_add]. + */ + smp_store_mb(current->jobctl, current->jobctl & ~JOBCTL_TASK_WORK); if (unlikely(current->task_works)) { spin_unlock_irq(&sighand->siglock); task_work_run(); --- a/kernel/task_work.c +++ b/kernel/task_work.c @@ -42,7 +42,13 @@ task_work_add(struct task_struct *task, set_notify_resume(task); break; case TWA_SIGNAL: - if (lock_task_sighand(task, &flags)) { + /* + * Only grab the sighand lock if we don't already have some + * task_work pending. This pairs with the smp_store_mb() + * in get_signal(), see comment there. + */ + if (!(READ_ONCE(task->jobctl) & JOBCTL_TASK_WORK) && + lock_task_sighand(task, &flags)) { task->jobctl |= JOBCTL_TASK_WORK; signal_wake_up(task, 0); unlock_task_sighand(task, &flags);