NFSv4: Fix a sleep in atomic context in nfs4_callback_sequence()

We must drop the lock before we can sleep in referring_call_exists().
Reported-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Fixes: 045d2a6d ("NFSv4.1: Delay callback processing...")
Cc: stable@vger.kernel.org # v4.9+
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/callback_proc.c

@@ -442,11 +442,14 @@ validate_seqid(const struct nfs4_slot_table *tbl, const struct nfs4_slot *slot,
  * a match.  If the slot is in use and the sequence numbers match, the
  * client is still waiting for a response to the original request.
  */
-static bool referring_call_exists(struct nfs_client *clp,
+static int referring_call_exists(struct nfs_client *clp,
 				  uint32_t nrclists,
-				  struct referring_call_list *rclists)
+				  struct referring_call_list *rclists,
+				  spinlock_t *lock)
+	__releases(lock)
+	__acquires(lock)
 {
-	bool status = false;
+	int status = 0;
 	int i, j;
 	struct nfs4_session *session;
 	struct nfs4_slot_table *tbl;
@@ -469,8 +472,10 @@ static bool referring_call_exists(struct nfs_client *clp,
 
 		for (j = 0; j < rclist->rcl_nrefcalls; j++) {
 			ref = &rclist->rcl_refcalls[j];
+			spin_unlock(lock);
 			status = nfs4_slot_wait_on_seqid(tbl, ref->rc_slotid,
 					ref->rc_sequenceid, HZ >> 1) < 0;
+			spin_lock(lock);
 			if (status)
 				goto out;
 		}
@@ -547,7 +552,8 @@ __be32 nfs4_callback_sequence(void *argp, void *resp,
 	 * related callback was received before the response to the original
 	 * call.
 	 */
-	if (referring_call_exists(clp, args->csa_nrclists, args->csa_rclists)) {
+	if (referring_call_exists(clp, args->csa_nrclists, args->csa_rclists,
+				&tbl->slot_tbl_lock) < 0) {
 		status = htonl(NFS4ERR_DELAY);
 		goto out_unlock;
 	}