linux-kernel - [04/89] futex_lock

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Date:	Tue, 30 Mar 2010 15:57:06 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Mikael Pettersson <mikpe@...uu.se>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Darren Hart <dvhltc@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Greg Kroah-Hartman <gregkh@...e.de>
Subject: [04/89] futex_lock_pi() key refcnt fix

2.6.31-stable review patch.  If anyone has any objections, please let us know.

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

From: Mikael Pettersson <mikpe@...uu.se>

commit 5ecb01cfdf96c5f465192bdb2a4fd4a61a24c6cc upstream.

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>
Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Acked-by: Darren Hart <dvhltc@...ibm.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

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

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1974,7 +1974,7 @@ 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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