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=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 C03AFC433DB for ; Fri, 12 Mar 2021 19:26:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5DA7464F85 for ; Fri, 12 Mar 2021 19:26:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233733AbhCLTZq (ORCPT ); Fri, 12 Mar 2021 14:25:46 -0500 Received: from Galois.linutronix.de ([193.142.43.55]:49390 "EHLO galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233727AbhCLTZS (ORCPT ); Fri, 12 Mar 2021 14:25:18 -0500 From: Thomas Gleixner DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1615577116; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=5zLfYrI1ktdYvU0WYUSOt8cuz631ADpBZR0ROWuzBbA=; b=fC6Qbeg592YV/SLHSVFbZQR8KJBndBfVtlqO0loaiN8u6znqmN3K3Fet8cQzLFT7JkfYl3 5ROMJqYfzuSFVfDc4Gd01p+lWycGZjcv/xpAQXGQ+aYxNzhBJtB3EGPu2bFNmTclj7kwe/ oDv+aUb96fpVhebOVXM/ulXJJC+DmJIFTqOV1i+Npb/S5rIYex0ALXy5YBO/Cg9AOq/sU7 lVA44ylYCmPKj6rd6hW9sOh7EenSTgB8qMdlQNtVDvlg3o8ojfGEV7Vzb+GxgmAgGt4VBQ PgA79PJf1Jpb0Q3ZeKOsaXAw23dIfpG2BRn44cwfvmnOqpoHe26RViFq1KZQ8w== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1615577116; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=5zLfYrI1ktdYvU0WYUSOt8cuz631ADpBZR0ROWuzBbA=; b=nS7IeHhuyXuf91cSN15FRXxDK/WPRkgLYCYWv9/0VxLU1hbe10Dcb4P9nCb5CahYzxCSyZ TBzC3MER9npvDQBQ== To: Oleg Nesterov , Sebastian Andrzej Siewior Cc: LKML , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Matt Fleming , "Eric W. Biederman" Subject: Re: [patch V2 3/3] signal: Allow tasks to cache one sigqueue struct In-Reply-To: <20210312161830.GA27820@redhat.com> References: <20210311132036.228542540@linutronix.de> <20210311141704.424120350@linutronix.de> <20210312113540.7byffvc46cgj75ah@linutronix.de> <20210312161830.GA27820@redhat.com> Date: Fri, 12 Mar 2021 20:25:16 +0100 Message-ID: <87eegk2mzn.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 12 2021 at 17:18, Oleg Nesterov wrote: > On 03/12, Sebastian Andrzej Siewior wrote: >> >> On 2021-03-11 14:20:39 [+0100], Thomas Gleixner wrote: >> > --- a/kernel/signal.c >> > +++ b/kernel/signal.c >> > @@ -433,7 +433,11 @@ static struct sigqueue * >> > rcu_read_unlock(); >> > >> > if (override_rlimit || likely(sigpending <= task_rlimit(t, RLIMIT_SIGPENDING))) { >> > - q = kmem_cache_alloc(sigqueue_cachep, gfp_flags); >> > + /* Preallocation does not hold sighand::siglock */ >> > + if (sigqueue_flags || !t->sigqueue_cache) >> > + q = kmem_cache_alloc(sigqueue_cachep, gfp_flags); >> > + else >> > + q = xchg(&t->sigqueue_cache, NULL); >> >> Could it happen that two tasks saw t->sigqueue_cache != NULL, the first >> one got the pointer via xchg() and the second got NULL via xchg()? > > It is called with sighand::siglock held, we don't even need xchg(). Yes, it was me being lazy. Lemme open code it as it's actually resulting in a locked instruction. Thanks, tglx