linux-kernel - Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup

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Message-ID: <20180425200917.GZ4082@hirez.programming.kicks-ass.net>
Date:   Wed, 25 Apr 2018 22:09:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Gaurav Kohli <gkohli@...eaurora.org>
Cc:     tglx@...utronix.de, mpe@...erman.id.au, mingo@...nel.org,
        bigeasy@...utronix.de, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against
 wakeup

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(&current->pi_lock);
>  			__set_current_state(TASK_RUNNING);
> +			raw_spin_unlock(&current->pi_lock);
>  			preempt_enable();
>  			if (ht->park && td->status == HP_THREAD_ACTIVE) {
>  				BUG_ON(td->cpu != smp_processor_id());

Does that work for you?

But looking at this a bit more; don't we have the exact same problem
with the TASK_RUNNING store in the !ht->thread_should_run() case?
Suppose a ttwu() happens concurrently there, it can end up competing
against the TASK_INTERRUPTIBLE store, no?

Of course, that race is not fatal, we'll just end up going around the
loop once again I suppose. Maybe a comment there too?

			/*
			 * A similar race is possible here, but loosing
			 * the TASK_INTERRUPTIBLE store is harmless and
			 * will make us go around the loop once more.
			 */

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.