Btrfs: device_replace: fix deadlock for nocow case

commit cb7ab02156e4 cause a following deadlock found by
xfstests,btrfs/011:

Thread1 is commiting transaction which is blocked at
btrfs_scrub_pause().

Thread2 is calling btrfs_file_aio_write() which has held
inode's @i_mutex and commit transaction(blocked because
Thread1 is committing transaction).

Thread3 is copy_nocow_page worker which will also try to
hold inode @i_mutex, so thread3 will wait Thread1 finished.

Thread4 is waiting pending workers finished which will wait
Thread3 finished. So the problem is like this:

Thread1--->Thread4--->Thread3--->Thread2---->Thread1

Deadlock happens! we fix it by letting Thread1 go firstly,
which means we won't block transaction commit while we are
waiting pending workers finished.

Reported-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>

diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
index 51c342b..f2f8803 100644 (file)
--- a/fs/btrfs/scrub.c
+++ b/fs/btrfs/scrub.c
@@ -2686,10 +2686,23 @@ int scrub_enumerate_chunks(struct scrub_ctx *sctx,
 
                wait_event(sctx->list_wait,
                           atomic_read(&sctx->bios_in_flight) == 0);
-               atomic_set(&sctx->wr_ctx.flush_all_writes, 0);
+               atomic_inc(&fs_info->scrubs_paused);
+               wake_up(&fs_info->scrub_pause_wait);
+
+               /*
+                * must be called before we decrease @scrub_paused.
+                * make sure we don't block transaction commit while
+                * we are waiting pending workers finished.
+                */
                wait_event(sctx->list_wait,
                           atomic_read(&sctx->workers_pending) == 0);
-               scrub_blocked_if_needed(fs_info);
+               atomic_set(&sctx->wr_ctx.flush_all_writes, 0);
+
+               mutex_lock(&fs_info->scrub_lock);
+               __scrub_blocked_if_needed(fs_info);
+               atomic_dec(&fs_info->scrubs_paused);
+               mutex_unlock(&fs_info->scrub_lock);
+               wake_up(&fs_info->scrub_pause_wait);
 
                btrfs_put_block_group(cache);
                if (ret)