Fix race in vdev_initialize_thread

Before allowing new allocations to the metaslab we need to ensure
that any issued initializing writes have been synced.  Otherwise,
it's possible for metaslab_block_alloc() to allocate a range which
is about to be overwritten by an initializing IO.

Serapheim Dimitropoulos <[email protected]>
Reviewed-by: Richard Elling <[email protected]>
Reviewed-by: Tim Chase <[email protected]>
Reviewed-by: George Wilson <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #8461 

1 files changed, 7 insertions, 0 deletions

diff --git a/module/zfs/vdev_initialize.c b/module/zfs/vdev_initialize.c
index a69eca354..bec89a28e 100644
--- a/module/zfs/vdev_initialize.c
+++ b/module/zfs/vdev_initialize.c
@@ -628,6 +628,13 @@ vdev_initialize_thread(void *arg)
 
 		spa_config_exit(spa, SCL_CONFIG, FTAG);
 		error = vdev_initialize_ranges(vd, deadbeef);
+
+		/*
+		 * Wait for the outstanding IO to be synced to prevent
+		 * newly allocated blocks from being overwritten.
+		 */
+		txg_wait_synced(spa_get_dsl(spa), 0);
+
 		vdev_initialize_ms_unmark(msp);
 		spa_config_enter(spa, SCL_CONFIG, FTAG, RW_READER);