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From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: linux-kernel@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	stable@vger.kernel.org,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	juri.lelli@arm.com, bigeasy@linutronix.de, xlpang@redhat.com,
	rostedt@goodmis.org, mathieu.desnoyers@efficios.com,
	jdesfossez@efficios.com, dvhart@infradead.org,
	bristot@redhat.com, Thomas Gleixner <tglx@linutronix.de>,
	Zhen Lei <thunder.leizhen@huawei.com>,
	Joe Korty <joe.korty@concurrent-rt.com>
Subject: [PATCH 4.4 06/11] futex: Futex_unlock_pi() determinism
Date: Sun,  8 Aug 2021 09:22:41 +0200	[thread overview]
Message-ID: <20210808072217.541927115@linuxfoundation.org> (raw)
In-Reply-To: <20210808072217.322468704@linuxfoundation.org>

From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit bebe5b514345f09be2c15e414d076b02ecb9cce8 ]

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>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Acked-by: Joe Korty <joe.korty@concurrent-rt.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 kernel/futex.c |   24 +++++++++++-------------
 1 file changed, 11 insertions(+), 13 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1555,15 +1555,10 @@ static int wake_futex_pi(u32 __user *uad
 	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
@@ -3020,15 +3015,18 @@ 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);



  parent reply	other threads:[~2021-08-08  7:23 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-08  7:22 [PATCH 4.4 00/11] 4.4.280-rc1 review Greg Kroah-Hartman
2021-08-08  7:22 ` [PATCH 4.4 01/11] futex: Rename free_pi_state() to put_pi_state() Greg Kroah-Hartman
2021-08-08  7:22 ` [PATCH 4.4 02/11] futex: Cleanup refcounting Greg Kroah-Hartman
2021-08-08  7:22 ` [PATCH 4.4 03/11] futex,rt_mutex: Introduce rt_mutex_init_waiter() Greg Kroah-Hartman
2021-08-08  7:22 ` [PATCH 4.4 04/11] futex: Pull rt_mutex_futex_unlock() out from under hb->lock Greg Kroah-Hartman
2021-08-08  7:22 ` [PATCH 4.4 05/11] futex: Rework futex_lock_pi() to use rt_mutex_*_proxy_lock() Greg Kroah-Hartman
2021-08-08  7:22 ` Greg Kroah-Hartman [this message]
2021-08-08  7:22 ` [PATCH 4.4 07/11] rtmutex: Make wait_lock irq safe Greg Kroah-Hartman
2021-08-08  7:22 ` [PATCH 4.4 08/11] futex: Handle transient "ownerless" rtmutex state correctly Greg Kroah-Hartman
2021-08-08  7:22 ` [PATCH 4.4 09/11] futex: Avoid freeing an active timer Greg Kroah-Hartman
2021-08-08  7:22 ` [PATCH 4.4 10/11] futex,rt_mutex: Fix rt_mutex_cleanup_proxy_lock() Greg Kroah-Hartman
2021-08-08  7:22 ` [PATCH 4.4 11/11] rcu: Update documentation of rcu_read_unlock() Greg Kroah-Hartman
2021-08-08 16:00 ` [PATCH 4.4 00/11] 4.4.280-rc1 review Guenter Roeck
2021-08-10 15:44   ` Greg Kroah-Hartman
2021-08-09 14:14 ` Naresh Kamboju
  -- strict thread matches above, loose matches on Subject: below --
2021-08-02 13:46 [PATCH 4.4 00/11] Fix a potential infinite loop in RT futex-pi scenarios Zhen Lei
2021-08-02 13:46 ` [PATCH 4.4 06/11] futex: Futex_unlock_pi() determinism Zhen Lei

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