From: Henrik Austad <henrik@austad.us>
To: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
stable@vger.kernel.org, Henrik Austad <haustad@cisco.com>,
Peter Zijlstra <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>
Subject: [PATCH 14/17] futex: Futex_unlock_pi() determinism
Date: Fri, 9 Nov 2018 11:07:42 +0100 [thread overview]
Message-ID: <1541758065-10952-15-git-send-email-henrik@austad.us> (raw)
In-Reply-To: <1541758065-10952-1-git-send-email-henrik@austad.us>
From: Peter Zijlstra <peterz@infradead.org>
commit bebe5b514345f09be2c15e414d076b02ecb9cce8 upstream.
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>
Tested-by: Henrik Austad <haustad@cisco.com>
---
kernel/futex.c | 24 +++++++++++-------------
1 file changed, 11 insertions(+), 13 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c
index 1cc40dd..14d270e 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1395,15 +1395,10 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_
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
@@ -2807,15 +2802,18 @@ 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);
--
2.7.4
next prev parent reply other threads:[~2018-11-09 10:08 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-09 10:07 [PATCH 00/17] Backport rt/deadline crash and the ardous story of FUTEX_UNLOCK_PI to 4.4 Henrik Austad
2018-11-09 10:07 ` [PATCH 01/17] futex: Cleanup variable names for futex_top_waiter() Henrik Austad
2018-11-09 10:07 ` [PATCH 02/17] futex: Use smp_store_release() in mark_wake_futex() Henrik Austad
2018-11-09 10:07 ` [PATCH 03/17] futex: Remove rt_mutex_deadlock_account_*() Henrik Austad
2018-11-09 10:07 ` [PATCH 04/17] rtmutex: Make wait_lock irq safe Henrik Austad
2018-11-09 10:07 ` [PATCH 05/17] futex,rt_mutex: Provide futex specific rt_mutex API Henrik Austad
2018-11-09 10:07 ` [PATCH 06/17] futex: Change locking rules Henrik Austad
2018-11-09 10:07 ` [PATCH 07/17] futex: Cleanup refcounting Henrik Austad
2018-11-09 10:07 ` [PATCH 08/17] futex: Rework inconsistent rt_mutex/futex_q state Henrik Austad
2018-11-09 10:07 ` [PATCH 09/17] futex: Rename free_pi_state() to put_pi_state() Henrik Austad
2018-11-09 10:07 ` [PATCH 10/17] futex: Pull rt_mutex_futex_unlock() out from under hb->lock Henrik Austad
2018-11-09 10:07 ` [PATCH 11/17] futex,rt_mutex: Introduce rt_mutex_init_waiter() Henrik Austad
2018-11-09 10:07 ` [PATCH 12/17] futex,rt_mutex: Restructure rt_mutex_finish_proxy_lock() Henrik Austad
2018-11-09 10:07 ` [PATCH 13/17] futex: Rework futex_lock_pi() to use rt_mutex_*_proxy_lock() Henrik Austad
2018-11-09 10:07 ` Henrik Austad [this message]
2018-11-09 10:07 ` [PATCH 15/17] futex: Drop hb->lock before enqueueing on the rtmutex Henrik Austad
2018-11-09 10:07 ` [PATCH 16/17] rtmutex: Deboost before waking up the top waiter Henrik Austad
2018-11-09 10:07 ` [PATCH 17/17] sched/rtmutex/deadline: Fix a PI crash for deadline tasks Henrik Austad
2018-11-09 10:35 ` [PATCH 00/17] Backport rt/deadline crash and the ardous story of FUTEX_UNLOCK_PI to 4.4 Henrik Austad
2018-11-19 11:27 ` Henrik Austad
2018-12-14 7:18 ` Greg Kroah-Hartman
2018-12-14 7:36 ` Henrik Austad
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=1541758065-10952-15-git-send-email-henrik@austad.us \
--to=henrik@austad.us \
--cc=bigeasy@linutronix.de \
--cc=bristot@redhat.com \
--cc=dvhart@infradead.org \
--cc=gregkh@linuxfoundation.org \
--cc=haustad@cisco.com \
--cc=jdesfossez@efficios.com \
--cc=juri.lelli@arm.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mathieu.desnoyers@efficios.com \
--cc=peterz@infradead.org \
--cc=rostedt@goodmis.org \
--cc=stable@vger.kernel.org \
--cc=tglx@linutronix.de \
--cc=xlpang@redhat.com \
/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).