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Date:   Mon, 3 Jun 2019 14:44:01 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Qian Cai <cai@lca.pw>, akpm@linux-foundation.org, hch@lst.de,
        oleg@redhat.com, gkohli@codeaurora.org, mingo@redhat.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] block: fix a crash in do_task_dead()
Message-ID: <20190603124401.GB3463@hirez.programming.kicks-ass.net>
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On Mon, Jun 03, 2019 at 02:37:05PM +0200, Peter Zijlstra wrote:

> Anyway, Oleg, do you see anything blatantly buggered with this patch?
> 
> (the stats were already dodgy for rq-stats, this patch makes them dodgy
> for task-stats too)

It now also has concurrency on wakeup; but afaict that's harmless, we'll
get racing stores of p->state = TASK_RUNNING, much the same as if there
was a remote wakeup vs a wait-loop terminating early.

I suppose the tracepoint consumers might have to deal with some
artifacts there, but that's their problem.

> ---
>  kernel/sched/core.c | 38 ++++++++++++++++++++++++++++++++------
>  1 file changed, 32 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 102dfcf0a29a..474aa4c8e9d2 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1990,6 +1990,28 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>  	unsigned long flags;
>  	int cpu, success = 0;
>  
> +	if (p == current) {
> +		/*
> +		 * We're waking current, this means 'p->on_rq' and 'task_cpu(p)
> +		 * == smp_processor_id()'. Together this means we can special
> +		 * case the whole 'p->on_rq && ttwu_remote()' case below
> +		 * without taking any locks.
> +		 *
> +		 * In particular:
> +		 *  - we rely on Program-Order guarantees for all the ordering,
> +		 *  - we're serialized against set_special_state() by virtue of
> +		 *    it disabling IRQs (this allows not taking ->pi_lock).
> +		 */
> +		if (!(p->state & state))
> +			goto out;
> +
> +		success = 1;
> +		trace_sched_waking(p);
> +		p->state = TASK_RUNNING;
> +		trace_sched_woken(p);
> +		goto out;
> +	}
> +
>  	/*
>  	 * If we are going to wake up a thread waiting for CONDITION we
>  	 * need to ensure that CONDITION=1 done by the caller can not be
> @@ -1999,7 +2021,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>  	raw_spin_lock_irqsave(&p->pi_lock, flags);
>  	smp_mb__after_spinlock();
>  	if (!(p->state & state))
> -		goto out;
> +		goto unlock;
>  
>  	trace_sched_waking(p);
>  
> @@ -2029,7 +2051,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>  	 */
>  	smp_rmb();
>  	if (p->on_rq && ttwu_remote(p, wake_flags))
> -		goto stat;
> +		goto unlock;
>  
>  #ifdef CONFIG_SMP
>  	/*
> @@ -2089,12 +2111,16 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>  #endif /* CONFIG_SMP */
>  
>  	ttwu_queue(p, cpu, wake_flags);
> -stat:
> -	ttwu_stat(p, cpu, wake_flags);
> -out:
> +unlock:
>  	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
>  
> -	return success;
> +out:
> +	if (success) {
> +		ttwu_stat(p, cpu, wake_flags);
> +		return true;
> +	}
> +
> +	return false;
>  }
>  
>  /**