[PATCH 2.6.33-rc5] futex_lock_pi() key refcnt fix

From: Mikael Pettersson
Date: Sat Jan 23 2010 - 16:36:42 EST

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@xxxxxxxx>
Cc: <stable@xxxxxxxxxx>
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 */

- goto out;
+ goto out_put_key;

queue_unlock(&q, hb);
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