block: fix deadlock in bdrv_co_flush

The following commit
    commit 3ff2f67a7c24183fcbcfe1332e5223ac6f96438c
    Author: Evgeny Yakovlev <eyakovlev@virtuozzo.com>
    Date:   Mon Jul 18 22:39:52 2016 +0300
    block: ignore flush requests when storage is clean
has introduced a regression.

There is a problem that it is still possible for 2 requests to execute
in non sequential fashion and sometimes this results in a deadlock
when bdrv_drain_one/all are called for BDS with such stalled requests.

1. Current flushed_gen and flush_started_gen is 1.
2. Request 1 enters bdrv_co_flush to with write_gen 1 (i.e. the same
   as flushed_gen). It gets past flushed_gen != flush_started_gen and
   sets flush_started_gen to 1 (again, the same it was before).
3. Request 1 yields somewhere before exiting bdrv_co_flush
4. Request 2 enters bdrv_co_flush with write_gen 2. It gets past
   flushed_gen != flush_started_gen and sets flush_started_gen to 2.
5. Request 2 runs to completion and sets flushed_gen to 2
6. Request 1 is resumed, runs to completion and sets flushed_gen to 1.
   However flush_started_gen is now 2.

From here on out flushed_gen is always != to flush_started_gen and all
further requests will wait on flush_queue. This change replaces
flush_started_gen with an explicitly tracked active flush request.

Signed-off-by: Evgeny Yakovlev <eyakovlev@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Message-id: 1471457214-3994-2-git-send-email-den@openvz.org
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Fam Zheng <famz@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/block/io.c b/block/io.c
index d5493ba349..9c04086286 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2283,11 +2283,11 @@ int coroutine_fn bdrv_co_flush(BlockDriverState *bs)
     int current_gen = bs->write_gen;
 
     /* Wait until any previous flushes are completed */
-    while (bs->flush_started_gen != bs->flushed_gen) {
+    while (bs->active_flush_req != NULL) {
         qemu_co_queue_wait(&bs->flush_queue);
     }
 
-    bs->flush_started_gen = current_gen;
+    bs->active_flush_req = &req;
 
     /* Write back all layers by calling one driver function */
     if (bs->drv->bdrv_co_flush) {
@@ -2357,6 +2357,7 @@ flush_parent:
 out:
     /* Notify any pending flushes that we have completed */
     bs->flushed_gen = current_gen;
+    bs->active_flush_req = NULL;
     qemu_co_queue_restart_all(&bs->flush_queue);
 
     tracked_request_end(&req);