Commit bebe5b51 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Thomas Gleixner
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futex: Futex_unlock_pi() determinism 



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: default avatarPeter 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: default avatarThomas Gleixner <tglx@linutronix.de>
parent cfafcd11

kernel/futex.c

+11 −13
Original line number Diff line number Diff line
@@ -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;



		get_pi_state(pi_state);

		/*

		 * Grab a reference on the pi_state and drop hb->lock.

		 * Since modifying the wait_list is done while holding both

		 * hb->lock and wait_lock, holding either is sufficient to

		 * observe it.

		 *

		 * 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.

		 * 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.

		 */

		get_pi_state(pi_state);

		raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);

		spin_unlock(&hb->lock);



		ret = wake_futex_pi(uaddr, uval, pi_state);

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