From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752987AbeDXS0m (ORCPT ); Tue, 24 Apr 2018 14:26:42 -0400 Received: from merlin.infradead.org ([205.233.59.134]:46866 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752937AbeDXS0g (ORCPT ); Tue, 24 Apr 2018 14:26:36 -0400 Date: Tue, 24 Apr 2018 20:26:28 +0200 From: Peter Zijlstra To: Gaurav Kohli Cc: tglx@linutronix.de, mpe@ellerman.id.au, dzickus@redhat.com, mingo@kernel.org, bigeasy@linutronix.de, linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, Neeraj Upadhyay Subject: Re: [PATCH] kthread/smpboot: Serialize kthread parking against wakeup Message-ID: <20180424182628.GW4043@hirez.programming.kicks-ass.net> References: <1524562105-31026-1-git-send-email-gkohli@codeaurora.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1524562105-31026-1-git-send-email-gkohli@codeaurora.org> 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 Tue, Apr 24, 2018 at 02:58:25PM +0530, Gaurav Kohli wrote: > The control cpu thread which initiates hotplug calls kthread_park() > for hotplug thread and sets KTHREAD_SHOULD_PARK. After this control > thread wakes up the hotplug thread. There is a chance that wakeup > code sees the hotplug thread (running on AP core) in INTERRUPTIBLE > state, but sets its state to RUNNING after hotplug thread has entered > kthread_parkme() and changed its state to TASK_PARKED. This can result > in panic later on in kthread_unpark(), as it sees KTHREAD_IS_PARKED > flag set but fails to rebind the kthread, due to it being not in > TASK_PARKED state. Fix this, by serializing wakeup state change, > against state change before parking the kthread. > > Below is the possible race: > > Control thread Hotplug Thread > > kthread_park() > set KTHREAD_SHOULD_PARK > smpboot_thread_fn > set_current_state(TASK_INTERRUPTIBLE); > kthread_parkme > > wake_up_process() > > raw_spin_lock_irqsave(&p->pi_lock, flags); > if (!(p->state & state)) -> this will fail > goto out; > > __kthread_parkme > __set_current_state(TASK_PARKED); > > if (p->on_rq && ttwu_remote(p, wake_flags)) > ttwu_remote() > p->state = TASK_RUNNING; > schedule(); > > So to avoid this race, take pi_lock to serial state changes. > > Suggested-by: Pavankumar Kondeti > Co-developed-by: Neeraj Upadhyay > Signed-off-by: Neeraj Upadhyay > Signed-off-by: Gaurav Kohli > > diff --git a/kernel/smpboot.c b/kernel/smpboot.c > index 1650578..514b232 100644 > --- a/kernel/smpboot.c > +++ b/kernel/smpboot.c > @@ -121,7 +121,9 @@ static int smpboot_thread_fn(void *data) > } > > if (kthread_should_park()) { > + 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()); Note how in your scenario above you didn't actually need the TASK_RUNNING state; so how is this change going to fix anything? But yes, I suspect it is right, but it definitely needs a comment explaining wth we take that lock there. Like I said earlier, my brain is entirely fried for the day; but I'll have a try tomorrow.