From: tip-bot for Peter Zijlstra <tipbot@zytor.com>
To: linux-tip-commits@vger.kernel.org
Cc: tglx@linutronix.de, hpa@zytor.com, peterz@infradead.org,
mingo@kernel.org, linux-kernel@vger.kernel.org
Subject: [tip:locking/core] futex: Futex_unlock_pi() determinism
Date: Thu, 23 Mar 2017 11:24:57 -0700 [thread overview]
Message-ID: <tip-bebe5b514345f09be2c15e414d076b02ecb9cce8@git.kernel.org> (raw)
In-Reply-To: <20170322104152.112378812@infradead.org>
Commit-ID: bebe5b514345f09be2c15e414d076b02ecb9cce8
Gitweb: http://git.kernel.org/tip/bebe5b514345f09be2c15e414d076b02ecb9cce8
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Wed, 22 Mar 2017 11:35:59 +0100
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitDate: Thu, 23 Mar 2017 19:10:10 +0100
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: 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>
---
kernel/futex.c | 24 +++++++++++-------------
1 file changed, 11 insertions(+), 13 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c
index eecce7b..4cdc603 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -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);
next prev parent reply other threads:[~2017-03-23 18:27 UTC|newest]
Thread overview: 60+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-22 10:35 [PATCH -v6 00/13] The arduous story of FUTEX_UNLOCK_PI Peter Zijlstra
2017-03-22 10:35 ` [PATCH -v6 01/13] futex: Cleanup variable names for futex_top_waiter() Peter Zijlstra
2017-03-23 18:19 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-03-24 21:11 ` [PATCH -v6 01/13] " Darren Hart
2017-03-22 10:35 ` [PATCH -v6 02/13] futex: Use smp_store_release() in mark_wake_futex() Peter Zijlstra
2017-03-23 18:19 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-03-24 21:16 ` [PATCH -v6 02/13] " Darren Hart
2017-03-22 10:35 ` [PATCH -v6 03/13] futex: Remove rt_mutex_deadlock_account_*() Peter Zijlstra
2017-03-23 18:20 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-03-24 21:29 ` [PATCH -v6 03/13] " Darren Hart
2017-03-24 21:31 ` Darren Hart
2017-03-22 10:35 ` [PATCH -v6 04/13] futex,rt_mutex: Provide futex specific rt_mutex API Peter Zijlstra
2017-03-23 18:20 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-03-25 0:37 ` [PATCH -v6 04/13] " Darren Hart
2017-04-06 12:15 ` Peter Zijlstra
2017-04-06 17:02 ` Darren Hart
2017-04-05 15:02 ` Darren Hart
2017-04-06 12:17 ` Peter Zijlstra
2017-04-06 17:08 ` Darren Hart
2017-03-22 10:35 ` [PATCH -v6 05/13] futex: Change locking rules Peter Zijlstra
2017-03-23 18:21 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-04-05 21:18 ` [PATCH -v6 05/13] " Darren Hart
2017-04-06 12:28 ` Peter Zijlstra
2017-04-06 15:58 ` Joe Perches
2017-04-06 17:21 ` Darren Hart
2017-03-22 10:35 ` [PATCH -v6 06/13] futex: Cleanup refcounting Peter Zijlstra
2017-03-23 18:21 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-04-05 21:29 ` [PATCH -v6 06/13] " Darren Hart
2017-03-22 10:35 ` [PATCH -v6 07/13] futex: Rework inconsistent rt_mutex/futex_q state Peter Zijlstra
2017-03-23 18:22 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-04-05 21:58 ` [PATCH -v6 07/13] " Darren Hart
2017-03-22 10:35 ` [PATCH -v6 08/13] futex: Pull rt_mutex_futex_unlock() out from under hb->lock Peter Zijlstra
2017-03-23 18:22 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-04-05 23:52 ` [PATCH -v6 08/13] " Darren Hart
2017-04-06 12:42 ` Peter Zijlstra
2017-04-06 17:42 ` Darren Hart
2017-03-22 10:35 ` [PATCH -v6 09/13] futex,rt_mutex: Introduce rt_mutex_init_waiter() Peter Zijlstra
2017-03-23 18:23 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-04-05 23:57 ` [PATCH -v6 09/13] " Darren Hart
2017-03-22 10:35 ` [PATCH -v6 10/13] futex,rt_mutex: Restructure rt_mutex_finish_proxy_lock() Peter Zijlstra
2017-03-23 18:23 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-04-07 23:30 ` [PATCH -v6 10/13] " Darren Hart
2017-04-07 23:35 ` Darren Hart
2017-03-22 10:35 ` [PATCH -v6 11/13] futex: Rework futex_lock_pi() to use rt_mutex_*_proxy_lock() Peter Zijlstra
2017-03-23 18:24 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-04-08 0:55 ` [PATCH -v6 11/13] " Darren Hart
2017-04-10 15:51 ` alexander.levin
2017-04-10 16:03 ` Thomas Gleixner
2017-04-14 9:30 ` [tip:locking/core] futex: Avoid freeing an active timer tip-bot for Thomas Gleixner
2017-03-22 10:35 ` [PATCH -v6 12/13] futex: futex_unlock_pi() determinism Peter Zijlstra
2017-03-23 18:24 ` tip-bot for Peter Zijlstra [this message]
2017-04-08 1:27 ` Darren Hart
2017-03-22 10:36 ` [PATCH -v6 13/13] futex: futex_lock_pi() vs PREEMPT_RT_FULL Peter Zijlstra
2017-03-23 18:25 ` [tip:locking/core] futex: Drop hb->lock before enqueueing on the rtmutex tip-bot for Peter Zijlstra
2017-04-08 2:26 ` [PATCH -v6 13/13] futex: futex_lock_pi() vs PREEMPT_RT_FULL Darren Hart
2017-04-08 5:22 ` Mike Galbraith
2017-04-10 8:43 ` Sebastian Andrzej Siewior
2017-04-10 9:08 ` Peter Zijlstra
2017-04-10 16:05 ` Darren Hart
2017-03-24 1:45 ` [PATCH -v6 00/13] The arduous story of FUTEX_UNLOCK_PI Darren Hart
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=tip-bebe5b514345f09be2c15e414d076b02ecb9cce8@git.kernel.org \
--to=tipbot@zytor.com \
--cc=hpa@zytor.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-tip-commits@vger.kernel.org \
--cc=mingo@kernel.org \
--cc=peterz@infradead.org \
--cc=tglx@linutronix.de \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).