[PATCH AUTOSEL 4.19 097/100] nbd: fix possible sysfs duplicate warning

[Sasha Levin]

From: Xiubo Li <xiubli@xxxxxxxxxx>

[ Upstream commit 862488105b84ca744b3d8ff131e0fcfe10644be1 ]

1. nbd_put takes the mutex and drops nbd->ref to 0. It then does
idr_remove and drops the mutex.

2. nbd_genl_connect takes the mutex. idr_find/idr_for_each fails
to find an existing device, so it does nbd_dev_add.

3. just before the nbd_put could call nbd_dev_remove or not finished
totally, but if nbd_dev_add try to add_disk, we can hit:

debugfs: Directory 'nbd1' with parent 'block' already present!

This patch will make sure all the disk add/remove stuff are done
by holding the nbd_index_mutex lock.

Reported-by: Mike Christie <mchristi@xxxxxxxxxx>
Reviewed-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
Signed-off-by: Xiubo Li <xiubli@xxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 drivers/block/nbd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index bc2fa4e85f0ca..d445195945618 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -228,8 +228,8 @@ static void nbd_put(struct nbd_device *nbd)
 	if (refcount_dec_and_mutex_lock(&nbd->refs,
 					&nbd_index_mutex)) {
 		idr_remove(&nbd_index_idr, nbd->index);
-		mutex_unlock(&nbd_index_mutex);
 		nbd_dev_remove(nbd);
+		mutex_unlock(&nbd_index_mutex);
 	}
 }
 
-- 
2.20.1