Commit 65181d63 authored Jul 22, 2019 by Max Reitz

block: Dec. drained_end_counter before bdrv_wakeup



Decrementing drained_end_counter after bdrv_dec_in_flight() (which in
turn invokes bdrv_wakeup() and thus aio_wait_kick()) is not very clever.
We should decrement it beforehand, so that any waiting aio_poll() that
is woken by bdrv_dec_in_flight() sees the decremented
drained_end_counter.

Because the time window between decrementing drained_end_counter and
aio_wait_kick() is very small, I cannot supply a reliable regression
test.  However, running e.g. the /bdrv-drain/blockjob/iothread/drain_all
test in test-bdrv-drain has a small chance of hanging without this
patch (about 1/200 or so; it gets to nearly 100 % if you add e.g. an
fputc(' ', stderr); after the bdrv_dec_in_flight()).

Fixes: e037c09c
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190722133054.21781-2-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

parent 258867d1

block/io.c

+2 −3

Original line number	Diff line number	Diff line
		@@ -217,13 +217,12 @@ static void coroutine_fn bdrv_drain_invoke_entry(void *opaque)
		bs->drv->bdrv_co_drain_end(bs);
		}

		/* Set data->done before reading bs->wakeup. */
		/* Set data->done and decrement drained_end_counter before bdrv_wakeup() */
		atomic_mb_set(&data->done, true);
		bdrv_dec_in_flight(bs);

		if (!data->begin) {
		atomic_dec(data->drained_end_counter);
		}
		bdrv_dec_in_flight(bs);

		g_free(data);
		}