From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755095AbeDZJO6 (ORCPT ); Thu, 26 Apr 2018 05:14:58 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:50418 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754116AbeDZJOx (ORCPT ); Thu, 26 Apr 2018 05:14:53 -0400 Date: Thu, 26 Apr 2018 11:14:49 +0200 From: Peter Zijlstra To: "Kohli, Gaurav" Cc: tglx@linutronix.de, mpe@ellerman.id.au, mingo@kernel.org, bigeasy@linutronix.de, linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, Neeraj Upadhyay , Will Deacon Subject: Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup Message-ID: <20180426091449.GA4082@hirez.programming.kicks-ass.net> References: <1524645199-5596-1-git-send-email-gkohli@codeaurora.org> <20180425200917.GZ4082@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.9.3 (2018-01-21) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 26, 2018 at 09:34:36AM +0530, Kohli, Gaurav wrote: > On 4/26/2018 1:39 AM, Peter Zijlstra wrote: > > > On Wed, Apr 25, 2018 at 02:03:19PM +0530, Gaurav Kohli wrote: > > > diff --git a/kernel/smpboot.c b/kernel/smpboot.c > > > index 5043e74..c5c5184 100644 > > > --- a/kernel/smpboot.c > > > +++ b/kernel/smpboot.c > > > @@ -122,7 +122,45 @@ static int smpboot_thread_fn(void *data) > > > } > > > if (kthread_should_park()) { > > > + /* > > > + * Serialize against wakeup. > > * > > * Prior wakeups must complete and later wakeups > > * will observe TASK_RUNNING. > > * > > * This avoids the case where the TASK_RUNNING > > * store from ttwu() competes with the > > * TASK_PARKED store from kthread_parkme(). > > * > > * If the TASK_PARKED store looses that > > * competition, kthread_unpark() will go wobbly. > > > + */ > > > + raw_spin_lock(¤t->pi_lock); > > > __set_current_state(TASK_RUNNING); > > > + raw_spin_unlock(¤t->pi_lock); > > > preempt_enable(); > > > if (ht->park && td->status == HP_THREAD_ACTIVE) { > > > BUG_ON(td->cpu != smp_processor_id()); > > Does that work for you? > > We have given patch for testing, usually it takes around 2-3 days for > reproduction(we will update for the same). I only changed the comment; surely your compiler doesn't generate different code for that? I was asking if the proposed comment was good with you; but see my more recent email, that actually proposes a different fix. > > /* > > * A similar race is possible here, but loosing > > * the TASK_INTERRUPTIBLE store is harmless and > > * will make us go around the loop once more. > > */ > > Actually instead of race, i am seeing wakeup miss problem which is > very rare, if we take case of hotplug thread Yes, triggering these issues is tricky, no doubt about that. > > And of course, I suspect we actually want to use TASK_IDLE, smpboot > > threads don't want signals do they? But that probably ought to be a > > separate patch. > > Yes I agree, we can control race from here as well,  Please suggest > would below change be any help here: That is not what I suggested. I said the thing should use TASK_IDLE instead of TASK_INTERRUPTIBLE. Not change the location of it.