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=-5.5 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 3DAFAC433DF for ; Tue, 11 Aug 2020 16:45:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1A59220768 for ; Tue, 11 Aug 2020 16:45:28 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=kernel-dk.20150623.gappssmtp.com header.i=@kernel-dk.20150623.gappssmtp.com header.b="mgi4l9Mk" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729275AbgHKQp0 (ORCPT ); Tue, 11 Aug 2020 12:45:26 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43526 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728844AbgHKQpX (ORCPT ); Tue, 11 Aug 2020 12:45:23 -0400 Received: from mail-pj1-x1041.google.com (mail-pj1-x1041.google.com [IPv6:2607:f8b0:4864:20::1041]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7C79EC06174A for ; Tue, 11 Aug 2020 09:45:23 -0700 (PDT) Received: by mail-pj1-x1041.google.com with SMTP id 2so2142682pjx.5 for ; Tue, 11 Aug 2020 09:45:23 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel-dk.20150623.gappssmtp.com; s=20150623; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=+u6gnCeWgAnf6Un1K5qbQxm4MSNlFFVkrtxRYNTa4wA=; b=mgi4l9MkxY2nMl+KwzHQbt4JpX/02zn8co0NW5FFXilO9SwvsO05LvTi7fG1RjdmOB YCUlUr3YTTQZfLrBa1RZMuzMHrXgdMv6lHcxph51NbD5uGUNX++4s+Z5SKkfSXp90JSe MGVYJ9wvzIsQ1H3TVRQDF4hn7B/gistjc/HA/Q5+dYLYzhU6WUJQlxozUqCKP3FE75JE mU+nmSUHMFovBxkgy75iHqcQu8hcgneaVyM8zqPAseGfjL8k6wOVYodSMD+eanoC9ZDt +gyXbmZurJOeIB4PpWEe+I1LKRglSyzbxr42XG6HLzLKEM7RSi6MfE+jpIQG7ChnwEs1 yCTw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=+u6gnCeWgAnf6Un1K5qbQxm4MSNlFFVkrtxRYNTa4wA=; b=uRcSEtqvY4Qz8r3hoQpV8D8XhQtRXeAy61ReqAWeDLaxJkexcZ9l32lWPauzhSEZrQ dnA0yZkkqSGmDKvkqSNJVg0FNND6B7v96UbYdFc5bypaVK1+ig1nCiOZY069Dy4P7GJz 84N1KfDVuNnuBfArdhZRm1cElc1FvCfBE0qaAOhaEbQ9xSaiSx1NUD+aDu3z9+dFbNl0 e5SYxnu7ZbQpvd9Kr5RlFqwYts7iMAlLe6+ebO/JpVy6/tA7z7gvjnDltPTyc0lzfU8M 4DDZUEJ0+rRLAQ82i92aCHvvgxKdmxaCMybvDgis+bhwhFjPnNdgpZXO9XD9R8BFqTh8 pHYg== X-Gm-Message-State: AOAM530HLz42TveMqIflEHZyp0ecB/0/GLq15NIsEgk2peHLsfIYsFgb br+DPvgYXMw4Z5l2ptENjr0xbw== X-Google-Smtp-Source: ABdhPJy00Yztk47tY8iwgI/wcQ3FdlwCOjlsi/uot40hsR/MeI0V+U7fH9t0jsOf+VhOwobefIO4/A== X-Received: by 2002:a17:902:8307:: with SMTP id bd7mr1610841plb.159.1597164322967; Tue, 11 Aug 2020 09:45:22 -0700 (PDT) Received: from [192.168.1.182] ([66.219.217.173]) by smtp.gmail.com with ESMTPSA id nu14sm3335490pjb.19.2020.08.11.09.45.21 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 11 Aug 2020 09:45:22 -0700 (PDT) Subject: Re: [PATCH] task_work: only grab task signal lock when needed To: Oleg Nesterov Cc: "linux-kernel@vger.kernel.org" , Peter Zijlstra , Jann Horn References: <0028d3ea-4d05-405f-b457-75c83d381d89@kernel.dk> <20200811152310.GF21797@redhat.com> From: Jens Axboe Message-ID: <6b43293f-2052-2bd8-80ef-eb936fe19b65@kernel.dk> Date: Tue, 11 Aug 2020 10:45:21 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200811152310.GF21797@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 8/11/20 9:23 AM, Oleg Nesterov wrote: > On 08/11, Jens Axboe wrote: >> >> --- a/kernel/task_work.c >> +++ b/kernel/task_work.c >> @@ -42,7 +42,8 @@ task_work_add(struct task_struct *task, struct callback_head *work, int notify) >> set_notify_resume(task); >> break; >> case TWA_SIGNAL: >> - if (lock_task_sighand(task, &flags)) { >> + if (!(READ_ONCE(task->jobctl) & JOBCTL_TASK_WORK) && >> + lock_task_sighand(task, &flags)) { > > Aaaaah, sorry Jens, now I think this is racy. So I am glad I didn't add > this optimization into the initial version ;) > > It is possible that JOBCTL_TASK_WORK is set but ->task_works == NULL. Say, > task_work_add(TWA_SIGNAL) + task_work_cancel(), or the target task can call > task_work_run() before it enters get_signal(). > > And in this case another task_work_add(tsk, TWA_SIGNAL) can actually race > with get_signal() which does > > current->jobctl &= ~JOBCTL_TASK_WORK; > if (unlikely(current->task_works)) { > spin_unlock_irq(&sighand->siglock); > task_work_run(); > > nothing guarantees that get_signal() sees ->task_works != NULL. Probably > this is what Jann meant. > > We can probably add a barrier into get_signal() but I didn't sleep today, > I'll try to think tomorrow. Appreciate you looking into this! Would be pretty critical for me to get this working. -- Jens Axboe