From 65181d63817b33b10ecb1c418eb96c99e7cf8842 Mon Sep 17 00:00:00 2001 From: Max Reitz Date: Mon, 22 Jul 2019 15:30:53 +0200 Subject: [PATCH] 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: e037c09c78520cbdb6da7cfc6ad0256d5870b814 Signed-off-by: Max Reitz Message-id: 20190722133054.21781-2-mreitz@redhat.com Signed-off-by: Max Reitz --- block/io.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) diff --git a/block/io.c b/block/io.c index b89e155d21..06305c6ea6 100644 --- a/block/io.c +++ b/block/io.c @@ -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); }