[PATCH 5.1 032/115] bcache: only set BCACHE_DEV_WB_RUNNING when cached device attached

From: Greg Kroah-Hartman
Date: Mon Jun 17 2019 - 17:24:44 EST

From: Coly Li <colyli@xxxxxxx>

commit 1f0ffa67349c56ea54c03ccfd1e073c990e7411e upstream.

When people set a writeback percent via sysfs file,
current code directly sets BCACHE_DEV_WB_RUNNING to dc->disk.flags
and schedules kworker dc->writeback_rate_update.

If there is no cache set attached to, the writeback kernel thread is
not running indeed, running dc->writeback_rate_update does not make
sense and may cause NULL pointer deference when reference cache set
pointer inside update_writeback_rate().

This patch checks whether the cache set point (dc->disk.c) is NULL in
sysfs interface handler, and only set BCACHE_DEV_WB_RUNNING and
schedule dc->writeback_rate_update when dc->disk.c is not NULL (it
means the cache device is attached to a cache set).

This problem might be introduced from initial bcache commit, but
commit 3fd47bfe55b0 ("bcache: stop dc->writeback_rate_update properly")
changes part of the original code piece, so I add 'Fixes: 3fd47bfe55b0'
to indicate from which commit this patch can be applied.

Fixes: 3fd47bfe55b0 ("bcache: stop dc->writeback_rate_update properly")
Reported-by: BjÃrn Forsman <bjorn.forsman@xxxxxxxxx>
Signed-off-by: Coly Li <colyli@xxxxxxx>
Reviewed-by: BjÃrn Forsman <bjorn.forsman@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

drivers/md/bcache/sysfs.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

--- a/drivers/md/bcache/sysfs.c
+++ b/drivers/md/bcache/sysfs.c
@@ -431,8 +431,13 @@ STORE(bch_cached_dev)

+ /*
+ * Only set BCACHE_DEV_WB_RUNNING when cached device attached to
+ * a cache set, otherwise it doesn't make sense.
+ */
if (attr == &sysfs_writeback_percent)
- if (!test_and_set_bit(BCACHE_DEV_WB_RUNNING, &dc->disk.flags))
+ if ((dc->disk.c != NULL) &&
+ (!test_and_set_bit(BCACHE_DEV_WB_RUNNING, &dc->disk.flags)))
dc->writeback_rate_update_seconds * HZ);