bcache: fix race of writeback thread starting before complete initialization

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@linux.ewheeler.net>
Tested-by: Marc MERLIN <marc@merlins.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 8d0ead98eb6e..b411c73bfeb3 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1015,8 +1015,12 @@ 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);
@@ -1028,6 +1032,9 @@ 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);