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 5CE0EC433FE for ; Mon, 25 Apr 2022 18:34:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244444AbiDYShP (ORCPT ); Mon, 25 Apr 2022 14:37:15 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57554 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232253AbiDYShO (ORCPT ); Mon, 25 Apr 2022 14:37:14 -0400 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 06107344E5; Mon, 25 Apr 2022 11:34:10 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=5qVQxZ1OD07RpfMUmbcCjjbi8k9aOtyZiVsapMUQrfA=; b=qTe4B3JbHuwWwHywQg48ldn1cQ IDjbGLtRO/AxqE8azkJzUWMFgWYcxoytP/qDgB5TzeiJstYnpvRzexWsRX99/CmVv5Yi+FE4Lclba f3pL4zIuTy8S01Bsy+BYR4vzWgadpbORfts/rISRb11Ef15vbF+O2zZizepfO9MmidJm+/XbLA7vA XLsMresvSrOqAcu8yZzBmIZe7cyLWfIZoDjdSTuBmbRtSUslz7t+FRTCplUlchqcAP5i/qdxDfkT5 pC3iyPYt9FJ7NnSY2l34rXIxnSrGkTixIDUys96S8COO6ruC1mvoG9v/ELoTH8NYGNxlC5BHGJifs aChvu0DA==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=worktop.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1nj3Wq-008bBp-Cl; Mon, 25 Apr 2022 18:33:44 +0000 Received: by worktop.programming.kicks-ass.net (Postfix, from userid 1000) id E7CB4980C21; Mon, 25 Apr 2022 20:33:43 +0200 (CEST) Date: Mon, 25 Apr 2022 20:33:43 +0200 From: Peter Zijlstra To: Oleg Nesterov Cc: rjw@rjwysocki.net, mingo@kernel.org, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, mgorman@suse.de, ebiederm@xmission.com, bigeasy@linutronix.de, Will Deacon , linux-kernel@vger.kernel.org, tj@kernel.org, linux-pm@vger.kernel.org Subject: Re: [PATCH v2 2/5] sched,ptrace: Fix ptrace_check_attach() vs PREEMPT_RT Message-ID: <20220425183343.GM2731@worktop.programming.kicks-ass.net> References: <20220421150248.667412396@infradead.org> <20220421150654.817117821@infradead.org> <20220425143537.GA12412@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220425143537.GA12412@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 25, 2022 at 04:35:37PM +0200, Oleg Nesterov wrote: > On 04/21, Peter Zijlstra wrote: > > > > +static void clear_traced_quiesce(void) > > +{ > > + spin_lock_irq(¤t->sighand->siglock); > > + WARN_ON_ONCE(!(current->jobctl & JOBCTL_TRACED_QUIESCE)); > > This WARN_ON_ONCE() doesn't look right, the task can be killed right > after ptrace_stop() sets JOBCTL_TRACED | JOBCTL_TRACED_QUIESCE and > drops siglock. OK, will look at that. > > @@ -2290,14 +2303,26 @@ static int ptrace_stop(int exit_code, in > > /* > > * Don't want to allow preemption here, because > > * sys_ptrace() needs this task to be inactive. > > - * > > - * XXX: implement read_unlock_no_resched(). > > */ > > preempt_disable(); > > read_unlock(&tasklist_lock); > > - cgroup_enter_frozen(); > > + cgroup_enter_frozen(); // XXX broken on PREEMPT_RT !!! > > + > > + /* > > + * JOBCTL_TRACE_QUIESCE bridges the gap between > > + * set_current_state(TASK_TRACED) above and schedule() below. > > + * There must not be any blocking (specifically anything that > > + * touched ->saved_state on PREEMPT_RT) between here and > > + * schedule(). > > + * > > + * ptrace_check_attach() relies on this with its > > + * wait_task_inactive() usage. > > + */ > > + clear_traced_quiesce(); > > Well, I think it should be called earlier under tasklist_lock, > before preempt_disable() above. > > We need tasklist_lock to protect ->parent, debugger can be killed > and go away right after read_unlock(&tasklist_lock). > > Still trying to convince myself everything is right with > JOBCTL_STOPPED/TRACED ... Can't do it earlier, since cgroup_enter_frozen() can do spinlock (eg. use ->saved_state).