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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id F02CAC433F5 for ; Wed, 16 Mar 2022 08:23:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1354498AbiCPIYl (ORCPT ); Wed, 16 Mar 2022 04:24:41 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44362 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234799AbiCPIYk (ORCPT ); Wed, 16 Mar 2022 04:24:40 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 16097DF43 for ; Wed, 16 Mar 2022 01:23:24 -0700 (PDT) Date: Wed, 16 Mar 2022 09:23:19 +0100 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1647419001; 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=KufJwsnfI5CF+IvC8AFaW/fYYl7ZQPnkJ0/TGFXMWf4=; b=u3498QkUVfjcu0P9/or7uHAM2kFHX8yrHC3/Ad7Hr8PtWbpM1SeXJ9qcYuDk8TGlxds1l9 J0njii7Fuqd3+3worrsQXicjU/B8GCTEv3Rl8MY783bkTAXdA0b4FRf3gaoCp16F/6rWPt qfAO+zksAHwHZPCFHEJkxg8/d35qRF5uC5nLS0JsCyAjtIGh8326wqP/cY2vlkTTXI/TbK kdlX8+NCPfpd4WNWcSVRtVfvhreS6Me5ApE28NHTKDSX8Byhl0CWk+2609xHKvSOMX2zaK YyxTzXuFlh/yvxswsTeFINOAl+cSDRVFRNnChMAltf6aY/IV0GRjP4V0zJp0UA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1647419001; 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=KufJwsnfI5CF+IvC8AFaW/fYYl7ZQPnkJ0/TGFXMWf4=; b=rr70YkguRBrIGAO97ABRjUp0zDrOik+5WMNNmvkQECrFBjtcckXVVO6KvEFR2eLRbqY8Om qO9kX3x6nFof4nBQ== From: Sebastian Andrzej Siewior To: Oleg Nesterov Cc: linux-kernel@vger.kernel.org, Ben Segall , Daniel Bristot de Oliveira , Dietmar Eggemann , Ingo Molnar , Juri Lelli , Mel Gorman , Peter Zijlstra , Steven Rostedt , Thomas Gleixner , Vincent Guittot Subject: Re: [PATCH] ptrace: fix ptrace vs tasklist_lock race on PREEMPT_RT. Message-ID: References: <20220314185429.GA30364@redhat.com> <20220315142944.GA22670@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20220315142944.GA22670@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022-03-15 15:29:46 [+0100], Oleg Nesterov wrote: > > > > --- a/kernel/sched/core.c > > > > +++ b/kernel/sched/core.c > > > > @@ -3239,7 +3239,8 @@ unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state > > > > * is actually now running somewhere else! > > > > */ > > > > while (task_running(rq, p)) { > > > > - if (match_state && unlikely(READ_ONCE(p->__state) != match_state)) > > > > + if (match_state && > > > > + unlikely(!task_state_match_eq(p, match_state))) > > > > return 0; > > > > > > So wait_task_inactive() can return 0 but the task can run after that, right? > > > This is not what we want... > > > > Without checking both states you may never observe the requested state > > because it is set to TASK_RTLOCK_WAIT while waiting for a lock. Other > > than that, it may run briefly because it tries to acquire a lock or just > > acquired and this shouldn't be different from a task spinning on a lock. > > I don't understand. wait_task_inactive() is used to ensure that this task > doesn't and can't run again, until debugger resumes these tracee. > > Now. Unless I missed something, the tracee can leave CPU with saved_state > = TRACED (so task_state_match_eq() returns T) and wait_task_inactive() will > return. Then later the tracee will park in schedule again, yes. > > But, for example, what if debugger clears TIF_BLOCKSTEP in between, while > the tracee is running? Can't this race with __switch_to_xtra() ? If you describe like that, then it appears better to only look at ->state. Otherwise, yes, you would see the expected state in ->saved_state and the task might still be on the CPU. Even if it is not actually running/ on the runqueue, it could be the case if the lock has been made available shortly after. > Oleg. Sebastian