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vfs: dcache: fix deadlock in tree traversal 

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>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Miklos Szeredi 2012-09-17 22:23:30 +02:00 committed by Linus Torvalds
parent 6a3e3dbee6
commit 8110e16d42
1 changed files with 6 additions and 0 deletions
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6
fs/dcache.c
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@ -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;