From ce83ee57f6fa69280a0330f1b49ef75b0ef0742a Mon Sep 17 00:00:00 2001 From: Evgeny Yakovlev Date: Wed, 17 Aug 2016 21:06:53 +0300 Subject: [PATCH] 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 +++-- include/block/block_int.h | 2 +- 2 files changed, 4 insertions(+), 3 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); diff --git a/include/block/block_int.h b/include/block/block_int.h index 47665be81e..1e939de4fe 100644 --- a/include/block/block_int.h +++ b/include/block/block_int.h @@ -443,8 +443,8 @@ struct BlockDriverState { note this is a reference count */ CoQueue flush_queue; /* Serializing flush queue */ + BdrvTrackedRequest *active_flush_req; /* Flush request in flight */ unsigned int write_gen; /* Current data generation */ - unsigned int flush_started_gen; /* Generation for which flush has started */ unsigned int flushed_gen; /* Flushed write generation */ BlockDriver *drv; /* NULL means no media */ -- 2.11.4.GIT