livelock avoidance in sget()

Eric Sandeen has found a nasty livelock in sget() - take a mount(2) about
to fail.  The superblock is on ->fs_supers, ->s_umount is held exclusive,
->s_active is 1.  Along comes two more processes, trying to mount the same
thing; sget() in each is picking that superblock, bumping ->s_count and
trying to grab ->s_umount.  ->s_active is 3 now.  Original mount(2)
finally gets to deactivate_locked_super() on failure; ->s_active is 2,
superblock is still ->fs_supers because shutdown will *not* happen until
->s_active hits 0.  ->s_umount is dropped and now we have two processes
chasing each other:
s_active = 2, A acquired ->s_umount, B blocked
A sees that the damn thing is stillborn, does deactivate_locked_super()
s_active = 1, A drops ->s_umount, B gets it
A restarts the search and finds the same superblock.  And bumps it ->s_active.
s_active = 2, B holds ->s_umount, A blocked on trying to get it
... and we are in the earlier situation with A and B switched places.

The root cause, of course, is that ->s_active should not grow until we'd
got MS_BORN.  Then failing ->mount() will have deactivate_locked_super()
shut the damn thing down.  Fortunately, it's easy to do - the key point
is that grab_super() is called only for superblocks currently on ->fs_supers,
so it can bump ->s_count and grab ->s_umount first, then check MS_BORN and
bump ->s_active; we must never increment ->s_count for superblocks past
->kill_sb(), but grab_super() is never called for those.

The bug is pretty old; we would've caught it by now, if not for accidental
exclusion between sget() for block filesystems; the things like cgroup or
e.g. mtd-based filesystems don't have anything of that sort, so they get
bitten.  The right way to deal with that is obviously to fix sget()...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 10 insertions, 15 deletions

diff --git a/fs/super.c b/fs/super.c
index 7465d4364208..68307c029228 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -336,19 +336,19 @@ EXPORT_SYMBOL(deactivate_super);
  *      and want to turn it into a full-blown active reference.  grab_super()
  *      is called with sb_lock held and drops it.  Returns 1 in case of
  *      success, 0 if we had failed (superblock contents was already dead or
- *      dying when grab_super() had been called).
+ *      dying when grab_super() had been called).  Note that this is only
+ *      called for superblocks not in rundown mode (== ones still on ->fs_supers
+ *      of their type), so increment of ->s_count is OK here.
  */
 static int grab_super(struct super_block *s) __releases(sb_lock)
 {
-        if (atomic_inc_not_zero(&s->s_active)) {
-                spin_unlock(&sb_lock);
-                return 1;
-        }
-        /* it's going away */
         s->s_count++;
         spin_unlock(&sb_lock);
-        /* wait for it to die */
         down_write(&s->s_umount);
+        if ((s->s_flags & MS_BORN) && atomic_inc_not_zero(&s->s_active)) {
+                put_super(s);
+                return 1;
+        }
         up_write(&s->s_umount);
         put_super(s);
         return 0;
@@ -463,11 +463,6 @@ retry:
                                 destroy_super(s);
                                 s = NULL;
                         }
-                        down_write(&old->s_umount);
-                        if (unlikely(!(old->s_flags & MS_BORN))) {
-                                deactivate_locked_super(old);
-                                goto retry;
-                        }
                         return old;
                 }
         }
@@ -660,10 +655,10 @@ restart:
                 if (hlist_unhashed(&sb->s_instances))
                         continue;
                 if (sb->s_bdev == bdev) {
-                        if (grab_super(sb)) /* drops sb_lock */
-                                return sb;
-                        else
+                        if (!grab_super(sb))
                                 goto restart;
+                        up_write(&sb->s_umount);
+                        return sb;
                 }
         }
         spin_unlock(&sb_lock);