cifs: add a warning if we try to to dequeue a deleted mid

cifs_delete_mid() is called once we are finished handling a mid and we
expect no more work done on this mid.

Needed to fix recent commit:
commit 730928c8f4be88e9d6a027a16b1e8fa9c59fc077
("cifs: update smb2_queryfs() to use compounding")

Add a warning if someone tries to dequeue a mid that has already been
flagged to be deleted.
Also change list_del() to list_del_init() so that if we have similar bugs
resurface in the future we will not oops.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>

fs/cifs/cifsglob.h

@@ -1553,6 +1553,7 @@ static inline void free_dfs_info_array(struct dfs_info3_param *param,
 
 /* Flags */
 #define   MID_WAIT_CANCELLED	 1 /* Cancelled while waiting for response */
+#define   MID_DELETED            2 /* Mid has been dequeued/deleted */
 
 /* Types of response buffer returned from SendReceive2 */
 #define   CIFS_NO_BUFFER        0    /* Response buffer not returned */

fs/cifs/connect.c

@@ -659,7 +659,15 @@ dequeue_mid(struct mid_q_entry *mid, bool malformed)
 		mid->mid_state = MID_RESPONSE_RECEIVED;
 	else
 		mid->mid_state = MID_RESPONSE_MALFORMED;
-	list_del_init(&mid->qhead);
+	/*
+	 * Trying to handle/dequeue a mid after the send_recv()
+	 * function has finished processing it is a bug.
+	 */
+	if (mid->mid_flags & MID_DELETED)
+		printk_once(KERN_WARNING
+			    "trying to dequeue a deleted mid\n");
+	else
+		list_del_init(&mid->qhead);
 	spin_unlock(&GlobalMid_Lock);
 }
 

fs/cifs/transport.c

@@ -142,7 +142,8 @@ void
 cifs_delete_mid(struct mid_q_entry *mid)
 {
 	spin_lock(&GlobalMid_Lock);
-	list_del(&mid->qhead);
+	list_del_init(&mid->qhead);
+	mid->mid_flags |= MID_DELETED;
 	spin_unlock(&GlobalMid_Lock);
 
 	DeleteMidQEntry(mid);