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Date:   Tue, 24 Sep 2019 13:43:00 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Pavel Begunkov <asml.silence@gmail.com>
Cc:     Jens Axboe <axboe@kernel.dk>, Ingo Molnar <mingo@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 0/2] Optimise io_uring completion waiting
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On Tue, Sep 24, 2019 at 02:11:29PM +0300, Pavel Begunkov wrote:

> @@ -2717,15 +2757,18 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
>  			return ret;
>  	}
>  
> +	iowq.nr_timeouts = atomic_read(&ctx->cq_timeouts);
> +	prepare_to_wait_exclusive(&ctx->wait, &iowq.wq, TASK_INTERRUPTIBLE);
> +	do {
> +		if (io_should_wake(&iowq))
> +			break;
> +		schedule();
> +		if (signal_pending(current))
> +			break;
> +		set_current_state(TASK_INTERRUPTIBLE);
> +	} while (1);
> +	finish_wait(&ctx->wait, &iowq.wq);

It it likely OK, but for paranoia, I'd prefer this form:

	for (;;) {
		prepare_to_wait_exclusive(&ctx->wait, &iowq.wq, TASK_INTERRUPTIBLE);
		if (io_should_wake(&iowq))
			break;
		schedule();
		if (signal_pending(current))
			break;
	}
	finish_wait(&ctx->wait, &iowq.wq);

The thing is, if we ever succeed with io_wake_function() (that CPU
observes io_should_wake()), but when waking here, we do not observe
is_wake_function() and go sleep again, we might never wake up if we
don't put ourselves back on the wait-list again.