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=-13.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=unavailable 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 8D57EC43215 for ; Mon, 18 Nov 2019 15:08:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4807A2071B for ; Mon, 18 Nov 2019 15:08:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1574089739; bh=0lenPH3anhwsrkXNO7JnaoQlB+fOcMRLpVM4KOr9YRQ=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=psA6pk2H/FrsFJuaWeiO7hSUJZwjso2c2taJ/EBecZ5Yaw35yU4ej3dQpb4h/Wrsg 5XXakxeq6wLdlAa8tY7giJRQjOoWWeWhciFtePCOoocHQ4Qlf4k0i0ByBdIFiLN625 IUOjuw8sfv9xbPm+T/+cll/OFTFa5vgjbOx95nWk= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727111AbfKRPI6 (ORCPT ); Mon, 18 Nov 2019 10:08:58 -0500 Received: from mail.kernel.org ([198.145.29.99]:38874 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726654AbfKRPI5 (ORCPT ); Mon, 18 Nov 2019 10:08:57 -0500 Received: from paulmck-ThinkPad-P72.home (199-192-87-166.static.wiline.com [199.192.87.166]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id DFFE020643; Mon, 18 Nov 2019 15:08:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1574089736; bh=0lenPH3anhwsrkXNO7JnaoQlB+fOcMRLpVM4KOr9YRQ=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=xerJ/44EBZHjxetyp4+8jXqyAHxIMxOjto4sd1Dp9Sl3R1af/fLO3lDxVhI2aCO4A aEJwNRJFZM5Q6JhfSB2kNrgtcAKWs4sdWXlfObbXmaQ0ees8tcPdWWQqtDI1JklUkS xQ+fU3gd14Qc0XroRV0Hv6J80XvxsKUyHfxjYb5E= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 99BA33520BE9; Mon, 18 Nov 2019 07:08:56 -0800 (PST) Date: Mon, 18 Nov 2019 07:08:56 -0800 From: "Paul E. McKenney" To: Neeraj Upadhyay Cc: josh@joshtriplett.org, joel@joelfernandes.org, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, linux-kernel@vger.kernel.org, rcu@vger.kernel.org, pkondeti@codeaurora.org, prsood@codeaurora.org, gkohli@codeaurora.org Subject: Re: [PATCH] rcu: Fix missed wakeup of exp_wq waiters Message-ID: <20191118150856.GN2889@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <1573838894-23027-1-git-send-email-neeraju@codeaurora.org> <20191117213624.GB2889@paulmck-ThinkPad-P72> <0101016e7dd7b824-50600058-ab5e-44d8-8d24-94cf095f1783-000000@us-west-2.amazonses.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0101016e7dd7b824-50600058-ab5e-44d8-8d24-94cf095f1783-000000@us-west-2.amazonses.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 18, 2019 at 09:28:39AM +0000, Neeraj Upadhyay wrote: > Hi Paul, > > On 11/18/2019 3:06 AM, Paul E. McKenney wrote: > > On Fri, Nov 15, 2019 at 10:58:14PM +0530, Neeraj Upadhyay wrote: > > > For the tasks waiting in exp_wq inside exp_funnel_lock(), > > > there is a chance that they might be indefinitely blocked > > > in below scenario: > > > > > > 1. There is a task waiting on exp sequence 0b'100' inside > > > exp_funnel_lock(). > > > > > > _synchronize_rcu_expedited() > > > > This symbol went away a few versions back, but let's see how this > > plays out in current -rcu. > > > > Sorry; for us this problem is observed on 4.19 stable version; I had > checked against the -rcu code, and the relevant portions were present > there. > > > > s = 0b'100 > > > exp_funnel_lock() > > > wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3] > > > > All of the above could still happen if the expedited grace > > period number was zero (or a bit less) when that task invoked > > Yes > > > synchronize_rcu_expedited(). What is the relation, if any, > > between this task and "task1" below? Seems like you want them to > > be different tasks. > > > > This task is the one which is waiting for the expedited sequence, which > "task1" completes ("task1" holds the exp_mutex for it). "task1" would > wake up this task, on exp GP completion. > > > Does this task actually block, or is it just getting ready > > to block? Seems like you need it to have actually blocked. > > > > Yes, it actually blocked in wait queue. > > > > 2. The Exp GP completes and task (task1) holding exp_mutex queues > > > worker and schedules out. > > > > "The Exp GP" being the one that was initiated when the .expedited_sequence > > counter was zero, correct? (Looks that way below.) > > > Yes, correct. > > > > _synchronize_rcu_expedited() > > > s = 0b'100 > > > queue_work(rcu_gp_wq, &rew.rew_work) > > > wake_up_worker() > > > schedule() > > > > > > 3. kworker A picks up the queued work and completes the exp gp > > > sequence. > > > > > > rcu_exp_wait_wake() > > > rcu_exp_wait_wake() > > > rcu_exp_gp_seq_end(rsp) // rsp->expedited_sequence is incremented > > > // to 0b'100' > > > > > > 4. task1 does not enter wait queue, as sync_exp_work_done() returns true, > > > and releases exp_mutex. > > > > > > wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3], > > > sync_exp_work_done(rsp, s)); > > > mutex_unlock(&rsp->exp_mutex); > > > > So task1 is the one that initiated the expedited grace period that > > started when .expedited_sequence was zero, right? > > > > Yes, right. > > > > 5. Next exp GP completes, and sequence number is incremented: > > > > > > rcu_exp_wait_wake() > > > rcu_exp_wait_wake() > > > rcu_exp_gp_seq_end(rsp) // rsp->expedited_sequence = 0b'200' > > > > > > 6. As kworker A uses current expedited_sequence, it wakes up workers > > > from wrong wait queue index - it should have worken wait queue > > > corresponding to 0b'100' sequence, but wakes up the ones for > > > 0b'200' sequence. This results in task at step 1 indefinitely blocked. > > > > > > rcu_exp_wait_wake() > > > wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rsp->expedited_sequence) & 0x3]); > > > > So the issue is that the next expedited RCU grace period might > > have completed before the completion of the wakeups for the previous > > expedited RCU grace period, correct? Then expedited grace periods have > > Yes. Actually from the ftraces, I saw that next expedited RCU grace > period completed while kworker A was in D state, while waiting for > exp_wake_mutex. This led to kworker A using sequence 2 (instead of 1) for > its wake_up_all() call; so, task (point 1) was never woken up, as it was > waiting on wq index 1. > > > to have stopped to prevent any future wakeup from happening, correct? > > (Which would make it harder for rcutorture to trigger this, though it > > really does have code that attempts to trigger this sort of thing.) > > > > Is this theoretical in nature, or have you actually triggered it? > > If actually triggered, what did you do to make this happen? > > This issue, we had seen previously - 1 instance in May 2018 (on 4.9 kernel), > another instance in Nov 2018 (on 4.14 kernel), in our customer reported > issues. Both instances were in downstream drivers and we didn't have RCU > traces. Now 2 days back, it was reported on 4.19 kernel, with RCU traces > enabled, where it was observed in suspend scenario, where we are observing > "DPM device timeout" [1], as scsi device is stuck in > _synchronize_rcu_expedited(). > > schedule+0x70/0x90 > _synchronize_rcu_expedited+0x590/0x5f8 > synchronize_rcu+0x50/0xa0 > scsi_device_quiesce+0x50/0x120 > scsi_bus_suspend+0x70/0xe8 > dpm_run_callback+0x148/0x388 > __device_suspend+0x430/0x8a8 > > [1] > https://github.com/torvalds/linux/blob/master/drivers/base/power/main.c#L489 > > > What have you done to test the change? > > > > I have given this for testing; will share the results . Current analysis > and patch is based on going through ftrace and code review. OK, very good. Please include the failure information in the changelog of the next version of this patch. I prefer your original patch, that just uses "s", over the one below that moves the rcu_exp_gp_seq_end(). The big advantage of your original patch is that it allow more concurrency between a consecutive pair of expedited RCU grace periods. Plus it would not be easy to convince myself that moving rcu_exp_gp_seq_end() down is safe, so your original is also conceptually simpler with a more manageable state space. Please also add the WARN_ON(), though at first glance your change seems to have lost the wakeup. (But it is early, so maybe it is just that I am not yet fully awake.) Thanx, Paul > I was thinking of another way of addressing this problem: Doing exp seq end > inside exp_wake_mutex. This will also ensure that, if we extend the current > scenario and there are multiple expedited GP sequence, which have completed, > before exp_wake_mutex is held, we need to preserve the requirement of 3 wq > entries [2]. > > > [2] https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git/tree/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst?h=dev > > > @@ -595,8 +595,6 @@ static void rcu_exp_wait_wake(struct rcu_state *rsp, > unsigned long s) > struct rcu_node *rnp; > > synchronize_sched_expedited_wait(rsp); > - rcu_exp_gp_seq_end(rsp); > - trace_rcu_exp_grace_period(rsp->name, s, TPS("end")); > > /* > * Switch over to wakeup mode, allowing the next GP, but -only- the > @@ -604,6 +602,9 @@ static void rcu_exp_wait_wake(struct rcu_state *rsp, > unsigned long s) > */ > mutex_lock(&rsp->exp_wake_mutex); > > + rcu_exp_gp_seq_end(rsp); > + trace_rcu_exp_grace_period(rsp->name, s, TPS("end")); > + > > > > > (Using a WARN_ON() to check for the lower bits of the counter portion > > of rcu_state.expedited_sequence differing from the same bits of s > > would be one way to detect this problem.) > > > > Thanx, Paul > > > > I have also given the patch for this, for testing: > > static void rcu_exp_wait_wake(struct rcu_state *rsp, unsigned long s) > { > struct rcu_node *rnp; > + unsigned long exp_low; > + unsigned long s_low = rcu_seq_ctr(s) & 0x3; > > synchronize_sched_expedited_wait(rsp); > rcu_exp_gp_seq_end(rsp); > @@ -613,7 +615,9 @@ static void rcu_exp_wait_wake(struct rcu_state *rsp, > unsigned long s) > spin_unlock(&rnp->exp_lock); > } > smp_mb(); /* All above changes before wakeup. */ > - wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rsp->expedited_sequence) & 0x3]); > + exp_low = rcu_seq_ctr(rsp->expedited_sequence) & 0x3; > + WARN_ON(s_low != exp_low); > + > > Thanks > Neeraj > > > > Fix this by using the correct sequence number for wake_up_all() inside > > > rcu_exp_wait_wake(). > > > > > > Signed-off-by: Neeraj Upadhyay > > > --- > > > kernel/rcu/tree_exp.h | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > > > index e4b77d3..28979d3 100644 > > > --- a/kernel/rcu/tree_exp.h > > > +++ b/kernel/rcu/tree_exp.h > > > @@ -557,7 +557,7 @@ static void rcu_exp_wait_wake(unsigned long s) > > > spin_unlock(&rnp->exp_lock); > > > } > > > smp_mb(); /* All above changes before wakeup. */ > > > - wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rcu_state.expedited_sequence) & 0x3]); > > > + wake_up_all(&rnp->exp_wq[rcu_seq_ctr(s) & 0x3]); > > > } > > > trace_rcu_exp_grace_period(rcu_state.name, s, TPS("endwake")); > > > mutex_unlock(&rcu_state.exp_wake_mutex); > > > -- > > > QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a > > > member of the Code Aurora Forum, hosted by The Linux Foundation > > > > > -- > QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of > the Code Aurora Forum, hosted by The Linux Foundation