[PATCH] writeback: fix NULL dereference when device is gone

[Peter Wu <lekensteyn@gmail.com>]

From: Peter Wu <lekensteyn@gmail.com>

This solves a BUG followed by a lockup in the following case:

1. Connect device and look in dmesg for the address:

    [   40.034520] usb 6-2: Product: My Passport 0748

2. Write something:

    dd if=/dev/zero of=/dev/sdd bs=1M

3. Remove device:

    echo 1 > /sys/bus/usb/devices/6-2/remove

4. Machine locks up (because the work queue got stuck?).

Signed-off-by: Peter Wu <lekensteyn@gmail.com>
---
 fs/fs-writeback.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 68851ff..6e38a8b 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1007,7 +1007,8 @@ void bdi_writeback_workfn(struct work_struct *work)
 	struct backing_dev_info *bdi = wb->bdi;
 	long pages_written;
 
-	set_worker_desc("flush-%s", dev_name(bdi->dev));
+	if (bdi->dev)
+		set_worker_desc("flush-%s", dev_name(bdi->dev));
 	current->flags |= PF_SWAPWRITE;
 
 	if (likely(!current_is_workqueue_rescuer() ||
-- 
1.8.3.4