linux-kernel - [PATCH 2.6.33-rc5] futex_lock

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Message-ID: <19291.27613.146510.938088@pilspetsen.it.uu.se>
Date:	Sat, 23 Jan 2010 22:36:29 +0100
From:	Mikael Pettersson <mikpe@...uu.se>
To:	linux-kernel@...r.kernel.org
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: [PATCH 2.6.33-rc5] futex_lock_pi() key refcnt fix

This fixes a futex key reference count bug in futex_lock_pi(),
where a key's reference count is incremented twice but decremented
only once, causing the backing object to not be released.

If the futex is created in a temporary file in an ext3 file system,
this bug causes the file's inode to become an "undead" orphan,
which causes an oops from a BUG_ON() in ext3_put_super() when the
file system is unmounted. glibc's test suite is known to trigger this,
see <http://bugzilla.kernel.org/show_bug.cgi?id=14256>.

The bug is a regression from 2.6.28-git3, namely Peter Zijlstra's
38d47c1b7075bd7ec3881141bb3629da58f88dab "[PATCH] futex: rely on
get_user_pages() for shared futexes". That commit made get_futex_key()
also increment the reference count of the futex key, and updated its
callers to decrement the key's reference count before returning.
Unfortunately the normal exit path in futex_lock_pi() wasn't corrected:
the reference count is incremented by get_futex_key() and queue_lock(),
but the normal exit path only decrements once, via unqueue_me_pi().
The fix is to put_futex_key() after unqueue_me_pi(), since 2.6.31
this is easily done by 'goto out_put_key' rather than 'goto out'.

Signed-off-by: Mikael Pettersson <mikpe@...uu.se>
Cc: <stable@...nel.org>
---
This patch also works for backports to 2.6.32 and 2.6.31,
but 2.6.30 and 2.6.29 need a "put_futex_key(fshared, &q.key);"
after "unqueue_me_pi(&q);" instead. I can supply patches
for these older kernels if necessary.

 kernel/futex.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff -rupN linux-2.6.33-rc5/kernel/futex.c linux-2.6.33-rc5.futex_lock_pi-key-refcnt-fix/kernel/futex.c
--- linux-2.6.33-rc5/kernel/futex.c	2010-01-22 01:21:57.000000000 +0100
+++ linux-2.6.33-rc5.futex_lock_pi-key-refcnt-fix/kernel/futex.c	2010-01-23 21:00:14.000000000 +0100
@@ -1971,7 +1971,7 @@ retry_private:
 	/* Unqueue and drop the lock */
 	unqueue_me_pi(&q);

-	goto out;
+	goto out_put_key;

 out_unlock_put_key:
 	queue_unlock(&q, hb);
--
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