From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Kohli, Gaurav" Subject: Re: [PATCH] kthread/smpboot: Serialize kthread parking against wakeup Date: Wed, 25 Apr 2018 00:16:19 +0530 Message-ID: <75ced7f3-e596-1942-f843-d43cf162103b@codeaurora.org> References: <1524562105-31026-1-git-send-email-gkohli@codeaurora.org> <20180424182628.GW4043@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: <20180424182628.GW4043@hirez.programming.kicks-ass.net> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Peter Zijlstra 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 List-Id: linux-arm-msm@vger.kernel.org On 4/24/2018 11:56 PM, Peter Zijlstra wrote: > 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? Hi Peter, As with help of this , if kthread_should_park run first so wake_up call of controller get exited as task is already set as running, otherwise if controller runs first then we will block here and set running and then sets TASK_PARKED . So no chance of cpuhp set as running during  kthread_parkme call. But as we discussed this can be fix by 2nd patch as well, So once you get time and able to see , Please let us know or do you want me to try your 2nd patch for testing first? > > 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. > -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.