From: Peter Zijlstra <peterz@infradead.org>
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: mingo@kernel.org, tglx@linutronix.de, juri.lelli@arm.com,
rostedt@goodmis.org, xlpang@redhat.com, bigeasy@linutronix.de,
linux-kernel@vger.kernel.org, mathieu.desnoyers@efficios.com,
jdesfossez@efficios.com, bristot@redhat.com
Subject: Re: [RFC][PATCH 2/4] futex: Use smp_store_release() in mark_wake_futex()
Date: Wed, 5 Oct 2016 08:20:30 +0200 [thread overview]
Message-ID: <20161005062030.GB3568@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <20161005035755.GC32728@linux-80c1.suse>
On Tue, Oct 04, 2016 at 08:57:55PM -0700, Davidlohr Bueso wrote:
> On Mon, 03 Oct 2016, Peter Zijlstra wrote:
>
> >Since the futex_q can dissapear the instruction after assigning NULL,
> >this really should be a RELEASE barrier. That stops loads from hitting
> >dead memory too.
> >
> >Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> >---
> >kernel/futex.c | 3 +--
> >1 file changed, 1 insertion(+), 2 deletions(-)
> >
> >--- a/kernel/futex.c
> >+++ b/kernel/futex.c
> >@@ -1288,8 +1288,7 @@ static void mark_wake_futex(struct wake_
> > * memory barrier is required here to prevent the following
> > * store to lock_ptr from getting ahead of the plist_del.
> > */
> >- smp_wmb();
> >- q->lock_ptr = NULL;
> >+ smp_store_release(&q->lock_ptr, NULL);
> >}
>
> Hmm, what if we relied on the implicit barrier in the wake_q_add()
> above and got rid of the smp_wmb altogether? We'd obviously have to
> move up __unqueue_futex(), but all we care about is the publishing
> store to lock_ptr being the last operation, or at least the plist_del,
> such that the wakeup order is respected; ie:
>
> __unqueue_futex(q);
> wake_q_add(wake_q, p);
> q->lock_ptr = NULL;
Less obvious but would work I suppose, I didn't look too hard at the
ordering requirements on __unqueue_futex(), but an early morning glance
(without tea) doesn't show any obvious problems with that.
next prev parent reply other threads:[~2016-10-05 6:20 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-10-03 9:12 [RFC][PATCH 0/4] FUTEX_UNLOCK_PI wobbles Peter Zijlstra
2016-10-03 9:12 ` [RFC][PATCH 1/4] futex: Cleanup variable names for futex_top_waiter() Peter Zijlstra
2016-10-03 14:15 ` Steven Rostedt
2016-10-05 3:58 ` Davidlohr Bueso
2016-10-03 9:12 ` [RFC][PATCH 2/4] futex: Use smp_store_release() in mark_wake_futex() Peter Zijlstra
2016-10-03 14:19 ` Steven Rostedt
2016-10-05 3:57 ` Davidlohr Bueso
2016-10-05 6:20 ` Peter Zijlstra [this message]
2016-10-03 9:12 ` [RFC][PATCH 3/4] futex: Remove rt_mutex_deadlock_account_*() Peter Zijlstra
2016-10-03 9:34 ` Peter Zijlstra
2016-10-03 14:25 ` Steven Rostedt
2016-10-05 1:08 ` Davidlohr Bueso
2016-10-05 7:29 ` Sebastian Andrzej Siewior
2016-10-03 9:12 ` [RFC][PATCH 4/4] futex: Rewrite FUTEX_UNLOCK_PI Peter Zijlstra
2016-10-03 15:36 ` Steven Rostedt
2016-10-03 15:44 ` Peter Zijlstra
2016-10-03 15:45 ` Peter Zijlstra
2016-10-03 16:23 ` Steven Rostedt
2016-10-05 7:41 ` Sebastian Andrzej Siewior
2016-10-05 8:09 ` Peter Zijlstra
2016-10-05 8:21 ` Sebastian Andrzej Siewior
2016-10-05 8:32 ` Peter Zijlstra
2016-10-06 10:29 ` Peter Zijlstra
2016-10-07 11:21 ` Peter Zijlstra
2016-10-08 15:53 ` Thomas Gleixner
2016-10-08 16:55 ` Peter Zijlstra
2016-10-08 17:06 ` Thomas Gleixner
2016-10-10 10:17 ` Thomas Gleixner
2016-10-10 11:40 ` Peter Zijlstra
2016-10-21 12:27 ` Peter Zijlstra
2016-10-27 20:36 ` Thomas Gleixner
2016-11-23 19:20 ` Peter Zijlstra
2016-11-24 16:52 ` Peter Zijlstra
2016-11-24 17:56 ` Thomas Gleixner
2016-11-24 18:58 ` Peter Zijlstra
2016-11-25 9:23 ` Peter Zijlstra
2016-11-25 10:03 ` Peter Zijlstra
2016-11-25 19:13 ` Thomas Gleixner
2016-11-25 14:09 ` Peter Zijlstra
2016-10-08 18:22 ` Thomas Gleixner
2016-10-09 11:17 ` Thomas Gleixner
2016-10-10 14:06 ` Peter Zijlstra
2016-10-05 1:02 ` [RFC][PATCH 0/4] FUTEX_UNLOCK_PI wobbles Davidlohr Bueso
2016-10-05 6:20 ` Peter Zijlstra
2016-10-05 7:26 ` Sebastian Andrzej Siewior
2016-10-05 16:04 ` Davidlohr Bueso
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=20161005062030.GB3568@worktop.programming.kicks-ass.net \
--to=peterz@infradead.org \
--cc=bigeasy@linutronix.de \
--cc=bristot@redhat.com \
--cc=dave@stgolabs.net \
--cc=jdesfossez@efficios.com \
--cc=juri.lelli@arm.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mathieu.desnoyers@efficios.com \
--cc=mingo@kernel.org \
--cc=rostedt@goodmis.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).