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Date:   Thu, 22 Nov 2018 12:13:51 +0100
From:   Jan Kara <jack@suse.cz>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     linux-block@vger.kernel.org, linux-aio@kvack.org,
        linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 8/8] aio: support for IO polling
Message-ID: <20181122111351.GD9840@quack2.suse.cz>
References: <20181120171953.1258-1-axboe@kernel.dk>
 <20181120171953.1258-9-axboe@kernel.dk>
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On Tue 20-11-18 10:19:53, Jens Axboe wrote:
> +/*
> + * We can't just wait for polled events to come to us, we have to actively
> + * find and complete them.
> + */
> +static void aio_iopoll_reap_events(struct kioctx *ctx)
> +{
> +	if (!(ctx->flags & IOCTX_FLAG_IOPOLL))
> +		return;
> +
> +	while (!list_empty_careful(&ctx->poll_submitted) ||
> +	       !list_empty(&ctx->poll_completing)) {
> +		unsigned int nr_events = 0;
> +
> +		__aio_iopoll_check(ctx, NULL, &nr_events, 1, UINT_MAX);
> +	}
> +}
> +
> +static int aio_iopoll_check(struct kioctx *ctx, long min_nr, long nr,
> +			    struct io_event __user *event)
> +{
> +	unsigned int nr_events = 0;
> +	int ret = 0;
> +
> +	/* * Only allow one thread polling at a time */
> +	if (test_and_set_bit(0, &ctx->getevents_busy))
> +		return -EBUSY;
> +
> +	while (!nr_events || !need_resched()) {
> +		int tmin = 0;
> +
> +		if (nr_events < min_nr)
> +			tmin = min_nr - nr_events;
> +
> +		ret = __aio_iopoll_check(ctx, event, &nr_events, tmin, nr);
> +		if (ret <= 0)
> +			break;
> +		ret = 0;
> +	}
> +
> +	clear_bit(0, &ctx->getevents_busy);
> +	return nr_events ? nr_events : ret;
> +}

Hum, what if userspace calls io_destroy() while another process is polling
for events on the same kioctx? It seems we'd be reaping events from two
processes in parallel in that case which will result in various
"interesting" effects like ctx->poll_completing list corruption...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR