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futex: Futex_unlock_pi() determinism

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The problem with returning -EAGAIN when the waiter state mismatches is that
it becomes very hard to proof a bounded execution time on the
operation. And seeing that this is a RT operation, this is somewhat
important.

While in practise; given the previous patch; it will be very unlikely to
ever really take more than one or two rounds, proving so becomes rather
hard.

However, now that modifying wait_list is done while holding both hb->lock
and wait_lock, the scenario can be avoided entirely by acquiring wait_lock
while still holding hb-lock. Doing a hand-over, without leaving a hole.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: juri.lelli@arm.com
Cc: bigeasy@linutronix.de
Cc: xlpang@redhat.com
Cc: rostedt@goodmis.org
Cc: mathieu.desnoyers@efficios.com
Cc: jdesfossez@efficios.com
Cc: dvhart@infradead.org
Cc: bristot@redhat.com
Link: http://lkml.kernel.org/r/20170322104152.112378812@infradead.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Peter Zijlstra 2017-03-22 11:35:59 +01:00 committed by Thomas Gleixner
parent cfafcd117d
commit bebe5b5143
1 changed files with 13 additions and 15 deletions
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28
kernel/futex.c
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@ -1398,15 +1398,10 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_
DEFINE_WAKE_Q(wake_q);
int ret = 0;
raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);
if (!new_owner) {
if (WARN_ON_ONCE(!new_owner)) {
/*
* Since we held neither hb->lock nor wait_lock when coming
* into this function, we could have raced with futex_lock_pi()
* such that we might observe @this futex_q waiter, but the
* rt_mutex's wait_list can be empty (either still, or again,
* depending on which side we land).
* As per the comment in futex_unlock_pi() this should not happen.
*
* When this happens, give up our locks and try again, giving
* the futex_lock_pi() instance time to complete, either by
@ -2794,15 +2789,18 @@ retry:
if (pi_state->owner != current)
goto out_unlock;
/*
* Grab a reference on the pi_state and drop hb->lock.
*
* The reference ensures pi_state lives, dropping the hb->lock
* is tricky.. wake_futex_pi() will take rt_mutex::wait_lock to
* close the races against futex_lock_pi(), but in case of
* _any_ fail we'll abort and retry the whole deal.
*/
get_pi_state(pi_state);
/*
* Since modifying the wait_list is done while holding both
* hb->lock and wait_lock, holding either is sufficient to
* observe it.
*
* By taking wait_lock while still holding hb->lock, we ensure
* there is no point where we hold neither; and therefore
* wake_futex_pi() must observe a state consistent with what we
* observed.
*/
raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
spin_unlock(&hb->lock);
ret = wake_futex_pi(uaddr, uval, pi_state);