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=-5.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 294B1C3A589 for ; Mon, 19 Aug 2019 01:42:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DECF721773 for ; Mon, 19 Aug 2019 01:42:02 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="CC2YO/5w" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726162AbfHSBmC (ORCPT ); Sun, 18 Aug 2019 21:42:02 -0400 Received: from mail-pf1-f196.google.com ([209.85.210.196]:45834 "EHLO mail-pf1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726103AbfHSBmC (ORCPT ); Sun, 18 Aug 2019 21:42:02 -0400 Received: by mail-pf1-f196.google.com with SMTP id w26so167197pfq.12 for ; Sun, 18 Aug 2019 18:42:02 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=GZ8KRd4m5tmf1sZH0S6i055h87V1Sn2Dsp8wrpuRW2I=; b=CC2YO/5wguKCQTET4BxoVFoyknrTC9mEtsaR0KfZra1yS6Z7NWVInFOefa/yn/TK5H a0zREvHq6w/5D6uDib9cB0lsf/PGWmE83+fQvDHmKsvcA+3t0W6OF8QwZ69IXqP/cIfq UFIhKICzQhQwPsNRBzFOzGebAou9XgBRqhA2w= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=GZ8KRd4m5tmf1sZH0S6i055h87V1Sn2Dsp8wrpuRW2I=; b=nrGq0aTWW3iJNI+Z4gqlVaMyeoLCVPaSlVeVACCSWrDBNZc3zO4GRCy79qV+MwbjU9 ufFb4TlHIUQ1iGXIGqCyegQVJfp5GqecYcVZULdhLpsbZ+J4F3kRm8m5XfLYNDFHAN8t zW/th/k0fjXaw9Ej0iJTMyt8B1AYz8xKvqy9mTt73kskS2H1NjboYp//dFxwgxVU5TTt Ns5GU+OxYEPlcud7UP7WxuXyRboajnJ/SuXgmDjZUOquDiY9MD2CssqdiqdrhLBkmNCC 5NHW9hQiI6fWWEMhAPN3P47s3pNJ/JkTM1UhreZl20KhZ2TFrMyEt9R/OYU4ntOhM8cu 13GA== X-Gm-Message-State: APjAAAXlttfrCp/AZn/i3goo8NBAZbUHqLfa2HVqlizPYspxggnOKdk9 +Bi+UdoG5UoWYGymEzSfnLt8aw== X-Google-Smtp-Source: APXvYqyEBa+u6Ju+6rey/3COmBroZZy8aLxdljJTHlK/Hd22pxrWiYNqZUqJY7j3uXws4p8YSZdzRw== X-Received: by 2002:a63:1b66:: with SMTP id b38mr6194396pgm.54.1566178921490; Sun, 18 Aug 2019 18:42:01 -0700 (PDT) Received: from localhost ([172.19.216.18]) by smtp.gmail.com with ESMTPSA id m20sm15034426pff.79.2019.08.18.18.42.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 18 Aug 2019 18:42:00 -0700 (PDT) Date: Sun, 18 Aug 2019 21:41:43 -0400 From: Joel Fernandes To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , rcu@vger.kernel.org, Steven Rostedt Subject: Re: [RFC v2] rcu/tree: Try to invoke_rcu_core() if in_irq() during unlock Message-ID: <20190819014143.GB160903@google.com> References: <20190818214948.GA134430@google.com> <20190818221210.GP28441@linux.ibm.com> <20190818223230.GA143857@google.com> <20190818223511.GB143857@google.com> <20190818233135.GQ28441@linux.ibm.com> <20190818233839.GA160903@google.com> <20190819012153.GR28441@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190819012153.GR28441@linux.ibm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Sun, Aug 18, 2019 at 06:21:53PM -0700, Paul E. McKenney wrote: > On Sun, Aug 18, 2019 at 07:38:39PM -0400, Joel Fernandes wrote: > > On Sun, Aug 18, 2019 at 04:31:35PM -0700, Paul E. McKenney wrote: > > > On Sun, Aug 18, 2019 at 06:35:11PM -0400, Joel Fernandes wrote: > > > > On Sun, Aug 18, 2019 at 06:32:30PM -0400, Joel Fernandes wrote: > > > > > On Sun, Aug 18, 2019 at 03:12:10PM -0700, Paul E. McKenney wrote: > > > > > > On Sun, Aug 18, 2019 at 05:49:48PM -0400, Joel Fernandes (Google) wrote: > > > > > > > When we're in hard interrupt context in rcu_read_unlock_special(), we > > > > > > > can still benefit from invoke_rcu_core() doing wake ups of rcuc > > > > > > > threads when the !use_softirq parameter is passed. This is safe > > > > > > > to do so because: > > > > > > > > > > > > > > 1. We avoid the scheduler deadlock issues thanks to the deferred_qs bit > > > > > > > introduced in commit 23634ebc1d94 ("rcu: Check for wakeup-safe > > > > > > > conditions in rcu_read_unlock_special()") by checking for the same in > > > > > > > this patch. > > > > > > > > > > > > > > 2. in_irq() implies in_interrupt() which implies raising softirq will > > > > > > > not do any wake ups. > > > > > > > > > > > > > > The rcuc thread which is awakened will run when the interrupt returns. > > > > > > > > > > > > > > We also honor 25102de ("rcu: Only do rcu_read_unlock_special() wakeups > > > > > > > if expedited") thus doing the rcuc awakening only when none of the > > > > > > > following are true: > > > > > > > 1. Critical section is blocking an expedited GP. > > > > > > > 2. A nohz_full CPU. > > > > > > > If neither of these cases are true (exp == false), then the "else" block > > > > > > > will run to do the irq_work stuff. > > > > > > > > > > > > > > This commit is based on a partial revert of d143b3d1cd89 ("rcu: Simplify > > > > > > > rcu_read_unlock_special() deferred wakeups") with an additional in_irq() > > > > > > > check added. > > > > > > > > > > > > > > Signed-off-by: Joel Fernandes (Google) > > > > > > > > > > > > OK, I will bite... If it is safe to wake up an rcuc kthread, why > > > > > > is it not safe to do raise_softirq()? > > > > > > > > > > Because raise_softirq should not be done and/or doesn't do anything > > > > > if use_softirq == false. In fact, RCU_SOFTIRQ doesn't even existing if > > > > > use_softirq == false. The "else if" condition of this patch uses for > > > > > use_softirq. > > > > > > > > > > Or, did I miss your point? > > > > > > I am concerned that added "else if" condition might not be sufficient > > > to eliminate all possible cases of the caller holding a scheduler lock, > > > which could result in deadlock in the ensuing wakeup. Might be me missing > > > something, but such deadlocks have been a recurring problem in the past. > > > > I thought that was the whole point of the > > rcu_read_unlock_special.b.deferred_qs bit that was introduced in > > 23634ebc1d94. We are checking that bit in the "else if" here as well. So this > > should be no less immune to scheduler deadlocks any more than the preceding > > "else if" where we are checking this bit. > > I would have much more confidence in a line of reasoning that led to > "immune to scheduler deadlocks" than one that led to "no less immune to > scheduler deadlocks". ;-) That is fair :-D But let me explain, What I meant is, if we are saying that this solution has a scheduler deadlock, then that would almost certainly imply that the existing code has scheduler deadlock issue. Since the existing code uses the same technique (using the deferred_qs bit in the union) to prevent the deadlock we were discussing a few months back. If that is indeed the case, it is good to be discussing this since we can discuss if the existing code needs any fixing as well. > > > Also, your commit log's point #2 is "in_irq() implies in_interrupt() > > > which implies raising softirq will not do any wake ups." This mention > > > of softirq seems a bit odd, given that we are going to wake up a rcuc > > > kthread. Of course, this did nothing to quell my suspicions. ;-) > > > > Yes, I should delete this #2 from the changelog since it is not very relevant > > (I feel now). My point with #2 was that even if were to raise a softirq > > (which we are not), a scheduler wakeup of ksoftirqd is impossible in this > > path anyway since in_irq() implies in_interrupt(). > > Please! Could you also add a first-principles explanation of why > the added condition is immune from scheduler deadlocks? Sure I can add an example in the change log, however I was thinking of this example which you mentioned: https://lore.kernel.org/lkml/20190627173831.GW26519@linux.ibm.com/ previous_reader() { rcu_read_lock(); do_something(); /* Preemption happened here. */ local_irq_disable(); /* Cannot be the scheduler! */ do_something_else(); rcu_read_unlock(); /* Must defer QS, task still queued. */ do_some_other_thing(); local_irq_enable(); } current_reader() /* QS from previous_reader() is still deferred. */ { local_irq_disable(); /* Might be the scheduler. */ do_whatever(); rcu_read_lock(); do_whatever_else(); rcu_read_unlock(); /* Must still defer reporting QS. */ do_whatever_comes_to_mind(); local_irq_enable(); } One modification of the example could be, previous_reader() could also do: previous_reader() { rcu_read_lock(); do_something_that_takes_really_long(); /* causes need_qs in the unlock_special_union to be set */ local_irq_disable(); /* Cannot be the scheduler! */ do_something_else(); rcu_read_unlock(); /* Must defer QS, task still queued. */ do_some_other_thing(); local_irq_enable(); } thanks! - Joel