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Date:   Mon, 3 Jun 2019 18:09:53 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Jens Axboe <axboe@kernel.dk>, Qian Cai <cai@lca.pw>,
        akpm@linux-foundation.org, hch@lst.de, 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: <20190603160953.GA15244@redhat.com>
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On 06/03, Peter Zijlstra wrote:
>
> 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.

I guess you mean that trace_sched_waking/wakeup can be reported twice if
try_to_wake_up(current) races with ttwu_remote(). And ttwu_stat().

> > --- 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);
                ^^^^^^^^^^^^^^^^^
trace_sched_wakeup(p) ?

I see nothing wrong... but probably this is because I don't fully understand
this change. In particular, I don't really understand who else can benefit from
this optimization...

Oleg.