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Message-ID: <4CBB17A8.70401@linux.intel.com>
Date: Sun, 17 Oct 2010 08:35:04 -0700
From: Darren Hart <dvhart@...ux.intel.com>
To: "lkml" <linux-kernel@...r.kernel.org>
CC: Matthieu Fertré <matthieu.fertre@...labs.com>,
Louis Rilling <louis.rilling@...labs.com>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...e.hu>,
Eric Dumazet <eric.dumazet@...il.com>,
John Kacur <jkacur@...hat.com>,
Rusty Russell <rusty@...tcorp.com.au>
Subject: [PATCH] futex: fix errors in nested key ref-counting
The following patch should address the ref counting issue reported by
Mattieu and Louis as well as one with futex_wait_requeue_pi. I have only
been able to test on a single socket dual core i7 system, I'd like to
see some additional testing.
I have another pair of patches which push the ref-counting out of
unqueue_me() and futex_wait_setup(), but they're exhibiting some
unexpected behavior. I didn't want to hold up this fix on those
cleanups.
>From 7b0ff7743691c07ef9283d63388d9cfc0de736ef Mon Sep 17 00:00:00 2001
From: Darren Hart <dvhart@...ux.intel.com>
Date: Fri, 15 Oct 2010 13:18:46 -0700
Subject: [PATCH] futex: fix errors in nested key ref-counting
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
futex_wait() was leaking key references due to futex_wait_setup() acquiring an
additional reference via the queue_lock() routine. The nested key ref-counting
has been masking bugs and complicating code analysis. queue_lock() is only
called with a previously ref-counted key, so remove the additional ref-counting
from the queue_(un)lock() functions.
futex_wait_requeue_pi() drops one key reference too many in unqueue_me_pi().
Remove the key reference handling from unqueue_me_pi(). This was paired with a
queue_lock() in futex_lock_pi(), so the count remains unchanged.
Document remaining nested key ref-counting sites.
Signed-off-by: Darren Hart <dvhart@...ux.intel.com>
Reported-by: Matthieu Fertré<matthieu.fertre@...labs.com>
Reported-by: Louis Rilling<louis.rilling@...labs.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...e.hu>
CC: Eric Dumazet <eric.dumazet@...il.com>
CC: John Kacur <jkacur@...hat.com>
CC: Rusty Russell <rusty@...tcorp.com.au>
---
kernel/futex.c | 31 ++++++++++++++++---------------
1 files changed, 16 insertions(+), 15 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c
index 6a3a5fa..abaafd0 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1363,7 +1363,6 @@ static inline struct futex_hash_bucket *queue_lock(struct futex_q *q)
{
struct futex_hash_bucket *hb;
- get_futex_key_refs(&q->key);
hb = hash_futex(&q->key);
q->lock_ptr = &hb->lock;
@@ -1375,7 +1374,6 @@ static inline void
queue_unlock(struct futex_q *q, struct futex_hash_bucket *hb)
{
spin_unlock(&hb->lock);
- drop_futex_key_refs(&q->key);
}
/**
@@ -1480,8 +1478,6 @@ static void unqueue_me_pi(struct futex_q *q)
q->pi_state = NULL;
spin_unlock(q->lock_ptr);
-
- drop_futex_key_refs(&q->key);
}
/*
@@ -1812,7 +1808,10 @@ static int futex_wait(u32 __user *uaddr, int fshared,
}
retry:
- /* Prepare to wait on uaddr. */
+ /*
+ * Prepare to wait on uaddr. On success, holds hb lock and increments
+ * q.key refs.
+ */
ret = futex_wait_setup(uaddr, val, fshared, &q, &hb);
if (ret)
goto out;
@@ -1822,24 +1821,23 @@ retry:
/* If we were woken (and unqueued), we succeeded, whatever. */
ret = 0;
+ /* unqueue_me() drops q.key ref */
if (!unqueue_me(&q))
- goto out_put_key;
+ goto out;
ret = -ETIMEDOUT;
if (to && !to->task)
- goto out_put_key;
+ goto out;
/*
* We expect signal_pending(current), but we might be the
* victim of a spurious wakeup as well.
*/
- if (!signal_pending(current)) {
- put_futex_key(fshared, &q.key);
+ if (!signal_pending(current))
goto retry;
- }
ret = -ERESTARTSYS;
if (!abs_time)
- goto out_put_key;
+ goto out;
restart = ¤t_thread_info()->restart_block;
restart->fn = futex_wait_restart;
@@ -1856,8 +1854,6 @@ retry:
ret = -ERESTART_RESTARTBLOCK;
-out_put_key:
- put_futex_key(fshared, &q.key);
out:
if (to) {
hrtimer_cancel(&to->timer);
@@ -2236,7 +2232,10 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
q.rt_waiter = &rt_waiter;
q.requeue_pi_key = &key2;
- /* Prepare to wait on uaddr. */
+ /*
+ * Prepare to wait on uaddr. On success, increments q.key (key1) ref
+ * count.
+ */
ret = futex_wait_setup(uaddr, val, fshared, &q, &hb);
if (ret)
goto out_key2;
@@ -2254,7 +2253,9 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
* In order for us to be here, we know our q.key == key2, and since
* we took the hb->lock above, we also know that futex_requeue() has
* completed and we no longer have to concern ourselves with a wakeup
- * race with the atomic proxy lock acquition by the requeue code.
+ * race with the atomic proxy lock acquition by the requeue code. The
+ * futex_requeue dropped our key1 reference and incremented our key2
+ * reference count.
*/
/* Check if the requeue code acquired the second futex for us. */
--
1.7.1
--
Darren Hart
Embedded Linux Kernel
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