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

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Date:	Sun, 07 Oct 2012 23:59:37 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Miklos Szeredi <miklos@...redi.hu>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Al Viro <viro@...IV.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [ 063/108] vfs: dcache: fix deadlock in tree traversal

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Miklos Szeredi <miklos@...redi.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@...e.cz>
Cc: Al Viro <viro@...IV.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.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;


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