From: Alan Stern <stern@rowland.harvard.edu>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>,
"Reshetova, Elena" <elena.reshetova@intel.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
"keescook@chromium.org" <keescook@chromium.org>,
"tglx@linutronix.de" <tglx@linutronix.de>,
"mingo@redhat.com" <mingo@redhat.com>,
"ishkamiel@gmail.com" <ishkamiel@gmail.com>,
Paul McKenney <paulmck@linux.vnet.ibm.com>,
<parri.andrea@gmail.com>, <boqun.feng@gmail.com>,
<dhowells@redhat.com>, <david@fromorbit.com>
Subject: Re: [PATCH] refcount: provide same memory ordering guarantees as in atomic_t
Date: Wed, 15 Nov 2017 15:22:33 -0500 (EST) [thread overview]
Message-ID: <Pine.LNX.4.44L0.1711151516380.1363-100000@iolanthe.rowland.org> (raw)
In-Reply-To: <20171115200307.ns4ja7xjwhunen65@hirez.programming.kicks-ass.net>
On Wed, 15 Nov 2017, Peter Zijlstra wrote:
> On Wed, Nov 15, 2017 at 02:15:19PM -0500, Alan Stern wrote:
> > On Wed, 15 Nov 2017, Will Deacon wrote:
> >
> > > On Thu, Nov 02, 2017 at 04:21:56PM -0400, Alan Stern wrote:
> > > > I was trying to think of something completely different. If you have a
> > > > release/acquire to the same address, it creates a happens-before
> > > > ordering:
> > > >
> > > > Access x
> > > > Release a
> > > > Acquire a
> > > > Access y
> > > >
> > > > Here is the access to x happens-before the access to y. This is true
> > > > even on x86, even in the presence of forwarding -- the CPU still has to
> > > > execute the instructions in order. But if the release and acquire are
> > > > to different addresses:
> > > >
> > > > Access x
> > > > Release a
> > > > Acquire b
> > > > Access y
> > > >
> > > > then there is no happens-before ordering for x and y -- the CPU can
> > > > execute the last two instructions before the first two. x86 and
> > > > PowerPC won't do this, but I believe ARMv8 can. (Please correct me if
> > > > it can't.)
> > >
> > > Release/Acquire are RCsc on ARMv8, so they are ordered irrespective of
> > > address.
> >
> > Ah, okay, thanks.
> >
> > In any case, we have considered removing this ordering constraint
> > (store-release followed by load-acquire for the same location) from the
> > Linux-kernel memory model.
>
> Why? Its a perfectly sensible construct.
>
> > I'm not aware of any code in the kernel that depends on it. Do any of
> > you happen to know of any examples?
>
> All locks? Something like:
>
> spin_lock(&x)
> /* foo */
> spin_unlock(&x)
> spin_lock(&x)
> /* bar */
> spin_unlock(&x);
>
> Has a fairly high foo happens-before bar expectation level.
>
> And in specific things like:
>
> 135e8c9250dd5
> ecf7d01c229d1
>
> which use the release of rq->lock paired with the next acquire of the
> same rq->lock to match with an smp_rmb().
You know, sometimes I feel like I'm losing my mind.
Yes, of course -- this was in fact the original reason for adding that
constraint to the memory model in the first place! An unlock-to-lock
link between two CPUs would naturally create an ordering relation, and
we wanted the same to be true when everything occurred on a single CPU.
I'll shut up now...
Alan
next prev parent reply other threads:[~2017-11-15 20:22 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-10-23 11:09 [PATCH] refcount: provide same memory ordering guarantees as in atomic_t Elena Reshetova
2017-10-23 13:12 ` Peter Zijlstra
2017-10-27 6:49 ` Reshetova, Elena
2017-10-27 13:56 ` Peter Zijlstra
2017-11-02 11:04 ` Reshetova, Elena
2017-11-02 13:57 ` Peter Zijlstra
2017-11-02 15:40 ` Alan Stern
2017-11-02 16:02 ` Peter Zijlstra
2017-11-02 16:45 ` Peter Zijlstra
2017-11-02 17:08 ` Alan Stern
2017-11-02 17:16 ` Will Deacon
2017-11-02 17:26 ` Peter Zijlstra
2017-11-02 20:21 ` Alan Stern
2017-11-15 18:05 ` Will Deacon
2017-11-15 19:15 ` Alan Stern
2017-11-15 20:03 ` Peter Zijlstra
2017-11-15 20:22 ` Alan Stern [this message]
2017-11-16 8:46 ` Peter Zijlstra
2017-11-15 21:01 ` Andrea Parri
2017-11-16 8:58 ` Peter Zijlstra
2017-11-16 10:00 ` Andrea Parri
2017-11-02 17:45 ` Andrea Parri
2017-11-02 20:28 ` Alan Stern
2017-11-03 11:55 ` Reshetova, Elena
2017-11-13 9:09 ` Reshetova, Elena
2017-11-13 13:19 ` Paul E. McKenney
2017-11-13 16:01 ` Reshetova, Elena
2017-11-13 16:26 ` Paul E. McKenney
2017-11-14 11:23 ` Reshetova, Elena
2017-11-14 17:24 ` Paul E. McKenney
2017-11-16 13:44 ` Michal Hocko
2017-11-16 15:29 ` Paul E. McKenney
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=Pine.LNX.4.44L0.1711151516380.1363-100000@iolanthe.rowland.org \
--to=stern@rowland.harvard.edu \
--cc=boqun.feng@gmail.com \
--cc=david@fromorbit.com \
--cc=dhowells@redhat.com \
--cc=elena.reshetova@intel.com \
--cc=gregkh@linuxfoundation.org \
--cc=ishkamiel@gmail.com \
--cc=keescook@chromium.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=parri.andrea@gmail.com \
--cc=paulmck@linux.vnet.ibm.com \
--cc=peterz@infradead.org \
--cc=tglx@linutronix.de \
--cc=will.deacon@arm.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).