drbd: fix potential deadlock when trying to detach during handshake

When requesting a detach, we first suspend IO, and also inhibit meta-data IO
by means of drbd_md_get_buffer(), because we don't want to "fail" the disk
while there is IO in-flight: the transition into D_FAILED for detach purposes
may get misinterpreted as actual IO error in a confused endio function.

We wrap it all into wait_event(), to retry in case the drbd_req_state()
returns SS_IN_TRANSIENT_STATE, as it does for example during an ongoing
connection handshake.

In that example, the receiver thread may need to grab drbd_md_get_buffer()
during the handshake to make progress.  To avoid potential deadlock with
detach, detach needs to grab and release the meta data buffer inside of
that wait_event retry loop. To avoid lock inversion between
mutex_lock(&device->state_mutex) and drbd_md_get_buffer(device),
introduce a new enum chg_state_flag CS_INHIBIT_MD_IO, and move the
call to drbd_md_get_buffer() inside the state_mutex grabbed in
drbd_req_state().

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 2 insertions, 23 deletions

diff --git a/drivers/block/drbd/drbd_nl.c b/drivers/block/drbd/drbd_nl.c
index c383b6cf272a..6bb58a6836ed 100644
--- a/drivers/block/drbd/drbd_nl.c
+++ b/drivers/block/drbd/drbd_nl.c
@@ -2149,34 +2149,13 @@ int drbd_adm_attach(struct sk_buff *skb, struct genl_info *info)
 
 static int adm_detach(struct drbd_device *device, int force)
 {
-	enum drbd_state_rv retcode;
-	void *buffer;
-	int ret;
-
 	if (force) {
 		set_bit(FORCE_DETACH, &device->flags);
 		drbd_force_state(device, NS(disk, D_FAILED));
-		retcode = SS_SUCCESS;
-		goto out;
+		return SS_SUCCESS;
 	}
 
-	drbd_suspend_io(device); /* so no-one is stuck in drbd_al_begin_io */
-	buffer = drbd_md_get_buffer(device, __func__); /* make sure there is no in-flight meta-data IO */
-	if (buffer) {
-		retcode = drbd_request_state(device, NS(disk, D_FAILED));
-		drbd_md_put_buffer(device);
-	} else /* already <= D_FAILED */
-		retcode = SS_NOTHING_TO_DO;
-	/* D_FAILED will transition to DISKLESS. */
-	drbd_resume_io(device);
-	ret = wait_event_interruptible(device->misc_wait,
-			device->state.disk != D_FAILED);
-	if ((int)retcode == (int)SS_IS_DISKLESS)
-		retcode = SS_NOTHING_TO_DO;
-	if (ret)
-		retcode = ERR_INTR;
-out:
-	return retcode;
+	return drbd_request_detach_interruptible(device);
 }
 
 /* Detaching the disk is a process in multiple stages.  First we need to lock