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Date: Fri, 2 Sep 2016 14:06:26 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>, Al Viro <viro@ZenIV.linux.org.uk>,
        Bart Van Assche <bvanassche@acm.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Neil Brown <neilb@suse.de>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] sched/wait: avoid abort_exclusive_wait() in
        __wait_on_bit_lock()
Message-ID: <20160902120626.GB26495@redhat.com>
References: <20160826124453.GA28894@redhat.com> <20160826124552.GB28904@redhat.com> <20160901190141.GJ10138@twins.programming.kicks-ass.net> <20160901190858.GI10168@twins.programming.kicks-ass.net>
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On 09/01, Peter Zijlstra wrote:
>
> > 	ret = 0;
> >
> > 	for (;;) {
> > 		prepare_to_wait_exclusive(wq, &q->wait, mode);
> >
> > 		if (test_bit(&q->key.bit_nr, &q->key.flag))
> > 			ret = action(&q->key, mode);
> >
> > 		if (!test_and_set_bit(&q->key.bit_nr, &q->key.flag)) {
> > 			/* we got the lock anyway, ignore the signal */
> > 			ret = 0;
> > 			break;
> > 		}
> >
> > 		if (ret)
> > 			break;
> > 	}
> > 	finish_wait(wq, &q->wait);
> >
> > 	return ret;
> >
> >
> > Would not that work too?
>
> Nope, because we need to do that finish_wait() before
> test_and_set_bit()..

Yes, I meant

	int __sched
	__wait_on_bit_lock(wait_queue_head_t *wq, struct wait_bit_queue *q,
				wait_bit_action_f *action, unsigned mode)
	{
		int ret = 0;

		for (;;) {
			prepare_to_wait_exclusive(wq, &q->wait, mode);
			if (test_bit(q->key.bit_nr, q->key.flags))
				ret = action(&q->key, mode);

			finish_wait(wq, &q->wait);

			if (!test_and_set_bit(q->key.bit_nr, q->key.flags))
				return 0;
			else if (ret)
				return ret;

		}
	}

> Also the problem with doing finish_wait() unconditionally would be
> destroying the FIFO order. With a bit of bad luck you'd get starvation
> cases :/

OK, I didn't think about that, thanks.

Oleg.