[PATCH v2 2/2] nbd: add comments about double lock for config_lock confusion

[xiubli@redhat.com]

From: Xiubo Li <xiubli@redhat.com>

When calling the ioctl(), fget() will be called on this fd, and
nbd_release() is only called when the fd's refcount drops to zero.
With this we can make sure that the nbd_release() won't be called
before the ioctl() finished.

So there won't have the double lock issue for the "config_lock",
which has already been held by nbd_ioctl().

Signed-off-by: Xiubo Li <xiubli@redhat.com>
---
 drivers/block/nbd.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 3bb8281bb753..48f36b003bf5 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -1345,6 +1345,17 @@ static void nbd_clear_sock_ioctl(struct nbd_device *nbd,
 	sock_shutdown(nbd);
 	__invalidate_device(bdev, true);
 	nbd_bdev_reset(bdev);
+
+	/*
+	 * When calling the ioctl(), fget() will be called on this
+	 * fd, and nbd_release() is only called when the fd's refcount
+	 * drops to zero. With this we can make sure that the
+	 * nbd_release() won't be called before the ioctl() finished.
+	 *
+	 * So there won't have the double lock issue if it will
+	 * call the nbd_config_put() here for the "config_lock", which
+	 * has already been held by nbd_ioctl().
+	 */
 	if (test_and_clear_bit(NBD_RT_HAS_CONFIG_REF,
 			       &nbd->config->runtime_flags))
 		nbd_config_put(nbd);
-- 
2.18.4