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SubjectRe: [PATCH 4/4] futex: convert hash_bucket locks to raw_spinlock_t
On Sat, 2010-07-10 at 21:41 +0200, Mike Galbraith wrote:
> On Fri, 2010-07-09 at 15:33 -0700, Darren Hart wrote:

> > If we can't move the unlock above before set_owner, then we may need a:
> >
> > retry:
> > cur->lock()
> > top_waiter = get_top_waiter()
> > cur->unlock()
> >
> > double_lock(cur, topwaiter)
> > if top_waiter != get_top_waiter()
> > double_unlock(cur, topwaiter)
> > goto retry
> >
> > Not ideal, but I think I prefer that to making all the hb locks raw.

Another option: only scratch the itchy spot.

futex: non-blocking synchronization point for futex_wait_requeue_pi() and futex_requeue().

Problem analysis by Darren Hart;
The requeue_pi mechanism introduced proxy locking of the rtmutex. This creates
a scenario where a task can wake-up, not knowing it has been enqueued on an
rtmutex. In order to detect this, the task would have to be able to take either
task->pi_blocked_on->lock->wait_lock and/or the hb->lock. Unfortunately,
without already holding one of these, the pi_blocked_on variable can change
from NULL to valid or from valid to NULL. Therefor, the task cannot be allowed
to take a sleeping lock after wakeup or it could end up trying to block on two
locks, the second overwriting a valid pi_blocked_on value. This obviously
breaks the pi mechanism.

Rather than convert the bh-lock to a raw spinlock, do so only in the spot where
blocking cannot be allowed, ie before we know that lock handoff has completed.

Signed-off-by: Mike Galbraith <>
Cc: Darren Hart <>
Cc: Thomas Gleixner <>
Cc: Peter Zijlstra <>
Cc: Ingo Molnar <>
Cc: Eric Dumazet <>
Cc: John Kacur <>
Cc: Steven Rostedt <>

diff --git a/kernel/futex.c b/kernel/futex.c
index a6cec32..ef489f3 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2255,7 +2255,14 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
/* Queue the futex_q, drop the hb lock, wait for wakeup. */
futex_wait_queue_me(hb, &q, to);

- spin_lock(&hb->lock);
+ /*
+ * Non-blocking synchronization point with futex_requeue().
+ *
+ * We dare not block here because this will alter PI state, possibly
+ * before our waker finishes modifying same in wakeup_next_waiter().
+ */
+ while(!spin_trylock(&hb->lock))
+ cpu_relax();
ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to);
if (ret)

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