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=-6.8 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 B2060C433E2 for ; Mon, 20 Jul 2020 11:26:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8AE6122B4D for ; Mon, 20 Jul 2020 11:26:33 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="XRdjG9aG" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728599AbgGTL0c (ORCPT ); Mon, 20 Jul 2020 07:26:32 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56218 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728058AbgGTL0b (ORCPT ); Mon, 20 Jul 2020 07:26:31 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id AD5A8C061794 for ; Mon, 20 Jul 2020 04:26:31 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; 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=QmcR/WJErNktzhfvR+IA0nw9TgotnNIvGmU7PfPVzvg=; b=XRdjG9aG2kS8rwH2hl32koil68 Dp8p+RYnmwsLnvrXWPUnHM9N7abUWnwea9XbLKuB2fY2dGgaaJmbSytclyWQgT++ZuTwp/ajcubKL xW/771Is76m7rAk6Ffu/wgLd5oFlqEXzz0aw1yPKk8mFfVD/+vHhSRFOujzc+KwQtc8Z3l9janFtB pi7HBZZY8iPautC6idjRjwcUmptbeGhUHnjj3z8O1OzVUFpffzsdyhTKvExk6Tf2N1ylw0b4MtzoS 5mfgqXBdwwliGo9fOZzkpaSnIODjCKaXTyH0Yn3YaBMB6kWXB1Bi5WfdrVJG1b4MTonR7g56n8BRO KwUkizXw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1jxTw8-0000Bv-1y; Mon, 20 Jul 2020 11:26:24 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 633C3301AC6; Mon, 20 Jul 2020 13:26:23 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 506ED200D4EAD; Mon, 20 Jul 2020 13:26:23 +0200 (CEST) Date: Mon, 20 Jul 2020 13:26:23 +0200 From: peterz@infradead.org To: Oleg Nesterov Cc: Jiri Slaby , Christian Brauner , christian@brauner.io, "Eric W. Biederman" , Linux kernel mailing list , Mel Gorman , Dave Jones , Paul Gortmaker Subject: Re: 5.8-rc*: kernel BUG at kernel/signal.c:1917 Message-ID: <20200720112623.GF43129@hirez.programming.kicks-ass.net> References: <20200717122651.GA6067@redhat.com> <20200717124017.GB6067@redhat.com> <2c8ef23c-43b4-39d4-8e84-92769c948da9@kernel.org> <20200718171406.GB16791@redhat.com> <20200718174448.4btbjcvp6wbbdgts@wittgenstein> <20200720064326.GA6612@redhat.com> <20200720082657.GC6612@redhat.com> <20200720084106.GJ10769@hirez.programming.kicks-ass.net> <20200720105924.GE43129@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200720105924.GE43129@hirez.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jul 20, 2020 at 12:59:24PM +0200, peterz@infradead.org wrote: > On Mon, Jul 20, 2020 at 10:41:06AM +0200, Peter Zijlstra wrote: > > On Mon, Jul 20, 2020 at 10:26:58AM +0200, Oleg Nesterov wrote: > > > Peter, > > > > > > Let me add another note. TASK_TRACED/TASK_STOPPED was always protected by > > > ->siglock. In particular, ttwu(__TASK_TRACED) must be always called with > > > ->siglock held. That is why ptrace_freeze_traced() assumes it can safely > > > do s/TASK_TRACED/__TASK_TRACED/ under spin_lock(siglock). > > > > > > Can this change race with > > > > > > if (signal_pending_state(prev->state, prev)) { > > > prev->state = TASK_RUNNING; > > > } > > > > > > in __schedule() ? Hopefully not, signal-state is protected by siglock too. > > > > > > So I think this logic was correct even if it doesn't look nice. But "doesn't > > > look nice" is true for the whole ptrace code ;) > > > > *groan*... another bit of obscure magic :-( > > > > let me go try and wake up and figure out how best to deal with this. This then? That seems to survive the strace thing. --- kernel/sched/core.c | 34 ++++++++++++++++++++++++++++------ 1 file changed, 28 insertions(+), 6 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index e15543cb84812..b5973d7fa521c 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4100,9 +4100,9 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) */ static void __sched notrace __schedule(bool preempt) { + unsigned long prev_state, tmp_state; struct task_struct *prev, *next; unsigned long *switch_count; - unsigned long prev_state; struct rq_flags rf; struct rq *rq; int cpu; @@ -4140,16 +4140,38 @@ static void __sched notrace __schedule(bool preempt) rq_lock(rq, &rf); smp_mb__after_spinlock(); + /* + * We must re-load prev->state in case ttwu_remote() changed it + * before we acquired rq->lock. + */ + tmp_state = prev->state; + if (unlikely(prev_state != tmp_state)) { + /* + * ptrace_{,un}freeze_traced() think it is cool to change + * ->state around behind our backs between TASK_TRACED and + * __TASK_TRACED. + * + * This is safe because this, as well as any __TASK_TRACED + * wakeups are under siglock. + * + * For any other case, a changed prev_state must be to + * TASK_RUNNING, such that when it blocks, the load has + * happened before the smp_mb(). + * + * Also see the comment with deactivate_task(). + */ + SCHED_WARN_ON(tmp_state && (prev_state & __TASK_TRACED && + !(tmp_state & __TASK_TRACED))); + + prev_state = tmp_state; + } + /* Promote REQ to ACT */ rq->clock_update_flags <<= 1; update_rq_clock(rq); switch_count = &prev->nivcsw; - /* - * We must re-load prev->state in case ttwu_remote() changed it - * before we acquired rq->lock. - */ - if (!preempt && prev_state && prev_state == prev->state) { + if (!preempt && prev_state) { if (signal_pending_state(prev_state, prev)) { prev->state = TASK_RUNNING; } else {