CIFS: Fix a deadlock when a file is reopened

If we request reading or writing on a file that needs to be
reopened, it causes the deadlock: we are already holding rw
semaphore for reading and then we try to acquire it for writing
in cifs_relock_file. Fix this by acquiring the semaphore for
reading in cifs_relock_file due to we don't make any changes in
locks and don't need a write access.

CC: <stable@vger.kernel.org>
Signed-off-by: Pavel Shilovsky <pshilovsky@samba.org>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/file.c

@@ -561,11 +561,10 @@ cifs_relock_file(struct cifsFileInfo *cfile)
 	struct cifs_tcon *tcon = tlink_tcon(cfile->tlink);
 	int rc = 0;
 
-	/* we are going to update can_cache_brlcks here - need a write access */
-	down_write(&cinode->lock_sem);
+	down_read(&cinode->lock_sem);
 	if (cinode->can_cache_brlcks) {
-		/* can cache locks - no need to push them */
-		up_write(&cinode->lock_sem);
+		/* can cache locks - no need to relock */
+		up_read(&cinode->lock_sem);
 		return rc;
 	}
 
@@ -576,7 +575,7 @@ cifs_relock_file(struct cifsFileInfo *cfile)
 	else
 		rc = tcon->ses->server->ops->push_mand_locks(cfile);
 
-	up_write(&cinode->lock_sem);
+	up_read(&cinode->lock_sem);
 	return rc;
 }