[PATCH 3.19.y-ckt 071/170] bcache: fix race of writeback thread starting before complete initialization

[Kamal Mostafa]

3.19.8-ckt18 -stable review patch.  If anyone has any objections, please let me know.

---8<------------------------------------------------------------

From: Eric Wheeler <git@xxxxxxxxxxxxxxxxxx>

commit 07cc6ef8edc47f8b4fc1e276d31127a0a5863d4d upstream.

The bch_writeback_thread might BUG_ON in read_dirty() if
dc->sb==BDEV_STATE_DIRTY and bch_sectors_dirty_init has not yet completed
its related initialization.  This patch downs the dc->writeback_lock until
after initialization is complete, thus preventing bch_writeback_thread
from proceeding prematurely.

See this thread:
  http://thread.gmane.org/gmane.linux.kernel.bcache.devel/3453

Signed-off-by: Eric Wheeler <bcache@xxxxxxxxxxxxxxxxxx>
Tested-by: Marc MERLIN <marc@xxxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
---
 drivers/md/bcache/super.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 42522c8..e83629a 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1046,8 +1046,12 @@ int bch_cached_dev_attach(struct cached_dev *dc, struct cache_set *c)
 	 */
 	atomic_set(&dc->count, 1);
 
-	if (bch_cached_dev_writeback_start(dc))
+	/* Block writeback thread, but spawn it */
+	down_write(&dc->writeback_lock);
+	if (bch_cached_dev_writeback_start(dc)) {
+		up_write(&dc->writeback_lock);
 		return -ENOMEM;
+	}
 
 	if (BDEV_STATE(&dc->sb) == BDEV_STATE_DIRTY) {
 		bch_sectors_dirty_init(dc);
@@ -1059,6 +1063,9 @@ int bch_cached_dev_attach(struct cached_dev *dc, struct cache_set *c)
 	bch_cached_dev_run(dc);
 	bcache_device_link(&dc->disk, c, "bdev");
 
+	/* Allow the writeback thread to proceed */
+	up_write(&dc->writeback_lock);
+
 	pr_info("Caching %s as %s on set %pU",
 		bdevname(dc->bdev, buf), dc->disk.disk->disk_name,
 		dc->disk.c->sb.set_uuid);
-- 
2.7.4