82 lines
3.2 KiB
Diff
82 lines
3.2 KiB
Diff
From: Peter Zijlstra <peterz@infradead.org>
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Date: Wed, 22 Mar 2017 11:35:59 +0100
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Subject: [PATCH] futex: Futex_unlock_pi() determinism
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Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.11/older/patches-4.11.5-rt1.tar.xz
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Upstream commit bebe5b514345f09be2c15e414d076b02ecb9cce8
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The problem with returning -EAGAIN when the waiter state mismatches is that
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it becomes very hard to proof a bounded execution time on the
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operation. And seeing that this is a RT operation, this is somewhat
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important.
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While in practise; given the previous patch; it will be very unlikely to
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ever really take more than one or two rounds, proving so becomes rather
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hard.
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However, now that modifying wait_list is done while holding both hb->lock
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and wait_lock, the scenario can be avoided entirely by acquiring wait_lock
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while still holding hb-lock. Doing a hand-over, without leaving a hole.
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Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
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Cc: juri.lelli@arm.com
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Cc: bigeasy@linutronix.de
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Cc: xlpang@redhat.com
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Cc: rostedt@goodmis.org
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Cc: mathieu.desnoyers@efficios.com
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Cc: jdesfossez@efficios.com
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Cc: dvhart@infradead.org
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Cc: bristot@redhat.com
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Link: http://lkml.kernel.org/r/20170322104152.112378812@infradead.org
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Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
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Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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---
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kernel/futex.c | 24 +++++++++++-------------
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1 file changed, 11 insertions(+), 13 deletions(-)
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--- a/kernel/futex.c
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+++ b/kernel/futex.c
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@@ -1398,15 +1398,10 @@ static int wake_futex_pi(u32 __user *uad
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DEFINE_WAKE_Q(wake_q);
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int ret = 0;
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- raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
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new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);
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- if (!new_owner) {
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+ if (WARN_ON_ONCE(!new_owner)) {
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/*
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- * Since we held neither hb->lock nor wait_lock when coming
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- * into this function, we could have raced with futex_lock_pi()
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- * such that we might observe @this futex_q waiter, but the
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- * rt_mutex's wait_list can be empty (either still, or again,
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- * depending on which side we land).
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+ * As per the comment in futex_unlock_pi() this should not happen.
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*
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* When this happens, give up our locks and try again, giving
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* the futex_lock_pi() instance time to complete, either by
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@@ -2794,15 +2789,18 @@ static int futex_unlock_pi(u32 __user *u
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if (pi_state->owner != current)
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goto out_unlock;
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+ get_pi_state(pi_state);
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/*
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- * Grab a reference on the pi_state and drop hb->lock.
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+ * Since modifying the wait_list is done while holding both
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+ * hb->lock and wait_lock, holding either is sufficient to
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+ * observe it.
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*
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- * The reference ensures pi_state lives, dropping the hb->lock
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- * is tricky.. wake_futex_pi() will take rt_mutex::wait_lock to
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- * close the races against futex_lock_pi(), but in case of
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- * _any_ fail we'll abort and retry the whole deal.
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+ * By taking wait_lock while still holding hb->lock, we ensure
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+ * there is no point where we hold neither; and therefore
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+ * wake_futex_pi() must observe a state consistent with what we
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+ * observed.
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*/
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- get_pi_state(pi_state);
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+ raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
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spin_unlock(&hb->lock);
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ret = wake_futex_pi(uaddr, uval, pi_state);
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