From ce83ee57f6fa69280a0330f1b49ef75b0ef0742a Mon Sep 17 00:00:00 2001 From: Evgeny Yakovlev Date: Wed, 17 Aug 2016 21:06:53 +0300 Subject: block: fix deadlock in bdrv_co_flush The following commit commit 3ff2f67a7c24183fcbcfe1332e5223ac6f96438c Author: Evgeny Yakovlev 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 Signed-off-by: Denis V. Lunev Message-id: 1471457214-3994-2-git-send-email-den@openvz.org CC: Stefan Hajnoczi CC: Fam Zheng CC: Kevin Wolf CC: Max Reitz Signed-off-by: Stefan Hajnoczi --- block/io.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) (limited to 'block') diff --git a/block/io.c b/block/io.c index d5493ba..9c04086 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); -- cgit v1.1