[ 063/108] vfs: dcache: fix deadlock in tree traversal

Ben Hutchings
2012-10-07T23:23:15Z

From: Miklos Szeredi <miklos@szeredi.hu>

commit 8110e16d42d587997bcaee0c864179e6d93603fe upstream.

IBM reported a deadlock in select_parent(). This was found to be caused
by taking rename_lock when already locked when restarting the tree
traversal.

There are two cases when the traversal needs to be restarted:

1) concurrent d_move(); this can only happen when not already locked,
since taking rename_lock protects against concurrent d_move().

2) racing with final d_put() on child just at the moment of ascending
to parent; rename_lock doesn't protect against this rare race, so it
can happen when already locked.

Because of case 2, we need to be able to handle restarting the traversal
when rename_lock is already held. This patch fixes all three callers of
try_to_ascend().

IBM reported that the deadlock is gone with this patch.

[ I rewrote the patch to be smaller and just do the "goto again" if the
lock was already held, but credit goes to Miklos for the real work.
- Linus ]

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/dcache.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/fs/dcache.c b/fs/dcache.c
index 0364af2..693f95b 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1134,6 +1134,8 @@ positive:
return 1;

rename_retry:
+ if (locked)
+ goto again;
locked = 1;
write_seqlock(&rename_lock);
goto again;
@@ -1236,6 +1238,8 @@ out:
rename_retry:
if (found)
return found;
+ if (locked)
+ goto again;
locked = 1;
write_seqlock(&rename_lock);
goto again;
@@ -3035,6 +3039,8 @@ resume:
return;

rename_retry:
+ if (locked)
+ goto again;
locked = 1;
write_seqlock(&rename_lock);
goto again;