mqueue: sys_mq_open: do not call mnt_drop_write() if read-only
Vladimir Davydov [Fri, 22 Mar 2013 22:04:51 +0000 (15:04 -0700)]
mnt_drop_write() must be called only if mnt_want_write() succeeded,
otherwise the mnt_writers counter will diverge.

mnt_writers counters are used to check if remounting FS as read-only is
OK, so after an extra mnt_drop_write() call, it would be impossible to
remount mqueue FS as read-only.  Besides, on umount a warning would be
printed like this one:

  =====================================
  [ BUG: bad unlock balance detected! ]
  3.9.0-rc3 #5 Not tainted
  -------------------------------------
  a.out/12486 is trying to release lock (sb_writers) at:
  mnt_drop_write+0x1f/0x30
  but there are no more locks to release!

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Doug Ledford <dledford@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/mqueue.c

index e5c4f60..3953fda 100644 (file)
@@ -840,7 +840,8 @@ out_putfd:
                fd = error;
        }
        mutex_unlock(&root->d_inode->i_mutex);
-       mnt_drop_write(mnt);
+       if (!ro)
+               mnt_drop_write(mnt);
 out_putname:
        putname(name);
        return fd;