From: Will Deacon <will.deacon@arm.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>,
Peter Zijlstra <peterz@infradead.org>,
Victor Kaplansky <VICTORK@il.ibm.com>,
Oleg Nesterov <oleg@redhat.com>,
Anton Blanchard <anton@samba.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Frederic Weisbecker <fweisbec@gmail.com>,
LKML <linux-kernel@vger.kernel.org>,
Linux PPC dev <linuxppc-dev@ozlabs.org>,
Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
Michael Ellerman <michael@ellerman.id.au>,
Michael Neuling <mikey@neuling.org>
Subject: Re: [RFC] arch: Introduce new TSO memory barrier smp_tmb()
Date: Mon, 4 Nov 2013 11:05:53 +0000 [thread overview]
Message-ID: <20131104110553.GA8595@mudshark.cambridge.arm.com> (raw)
In-Reply-To: <CA+55aFyD_kCkAHQwHCUBrumO-pH6LaZikTNvyWDW_tWsHdqk6Q@mail.gmail.com>
On Sun, Nov 03, 2013 at 11:34:00PM +0000, Linus Torvalds wrote:
> So it would *kind* of act like a "smp_wmb() + smp_rmb()", but the
> problem is that a "smp_rmb()" doesn't really "attach" to the preceding
> write.
Agreed.
> This is analogous to a "acquire" operation: you cannot make an
> "acquire" barrier, because it's not a barrier *between* two ops, it's
> associated with one particular op.
>
> So what I *think* you actually really really want is a "store with
> release consistency, followed by a write barrier".
How does that order reads against reads? (Paul mentioned this as a
requirement). I not clear about the use case for this, so perhaps there is a
dependency that I'm not aware of.
> In TSO, afaik all stores have release consistency, and all writes are
> ordered, which is why this is a no-op in TSO. And x86 also has that
> "all stores have release consistency, and all writes are ordered"
> model, even if TSO doesn't really describe the x86 model.
>
> But on ARM64, for example, I think you'd really want the store itself
> to be done with "stlr" (store with release), and then follow up with a
> "dsb st" after that.
So a dsb is pretty heavyweight here (it prevents execution of *any* further
instructions until all preceeding stores have completed, as well as
ensuring completion of any ongoing cache flushes). In conjunction with the
store-release, that's going to hold everything up until the store-release
(and therefore any preceeding memory accesses) have completed. Granted, I
think that gives Paul his read/read ordering, but it's a lot heavier than
what's required.
> And notice how that requires you to mark the store itself. There is no
> actual barrier *after* the store that does the optimized model.
>
> Of course, it's entirely possible that it's not worth worrying about
> this on ARM64, and that just doing it as a "normal store followed by a
> full memory barrier" is good enough. But at least in *theory* a
> microarchitecture might make it much cheaper to do a "store with
> release consistency" followed by "write barrier".
I agree with the sentiment but, given that this stuff is so heavily
microarchitecture-dependent (and not simple to probe), a simple dmb ish
might be the best option after all. That's especially true if the
microarchitecture decided to ignore the barrier options and treat everything
as `all accesses, full system' in order to keep the hardware design simple.
Will
next prev parent reply other threads:[~2013-11-04 11:07 UTC|newest]
Thread overview: 117+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-10-22 23:54 perf events ring buffer memory barrier on powerpc Michael Neuling
2013-10-23 7:39 ` Victor Kaplansky
2013-10-23 14:19 ` Frederic Weisbecker
2013-10-23 14:25 ` Frederic Weisbecker
2013-10-25 17:37 ` Peter Zijlstra
2013-10-25 20:31 ` Michael Neuling
2013-10-27 9:00 ` Victor Kaplansky
2013-10-28 9:22 ` Peter Zijlstra
2013-10-28 10:02 ` Frederic Weisbecker
2013-10-28 12:38 ` Victor Kaplansky
2013-10-28 13:26 ` Peter Zijlstra
2013-10-28 16:34 ` Paul E. McKenney
2013-10-28 20:17 ` Oleg Nesterov
2013-10-28 20:58 ` Victor Kaplansky
2013-10-29 10:21 ` Peter Zijlstra
2013-10-29 10:30 ` Peter Zijlstra
2013-10-29 10:35 ` Peter Zijlstra
2013-10-29 20:15 ` Oleg Nesterov
2013-10-29 19:27 ` Vince Weaver
2013-10-30 10:42 ` Peter Zijlstra
2013-10-30 11:48 ` James Hogan
2013-10-30 12:48 ` Peter Zijlstra
2013-11-06 13:19 ` [tip:perf/core] tools/perf: Add required memory barriers tip-bot for Peter Zijlstra
2013-11-06 13:50 ` Vince Weaver
2013-11-06 14:00 ` Peter Zijlstra
2013-11-06 14:28 ` Peter Zijlstra
2013-11-06 14:55 ` Vince Weaver
2013-11-06 15:10 ` Peter Zijlstra
2013-11-06 15:23 ` Peter Zijlstra
2013-11-06 14:44 ` Peter Zijlstra
2013-11-06 16:07 ` Peter Zijlstra
2013-11-06 17:31 ` Vince Weaver
2013-11-06 18:24 ` Peter Zijlstra
2013-11-07 8:21 ` Ingo Molnar
2013-11-07 14:27 ` Vince Weaver
2013-11-07 15:55 ` Ingo Molnar
2013-11-11 16:24 ` Peter Zijlstra
2013-11-11 21:10 ` Ingo Molnar
2013-10-29 21:23 ` perf events ring buffer memory barrier on powerpc Michael Neuling
2013-10-30 9:27 ` Paul E. McKenney
2013-10-30 11:25 ` Peter Zijlstra
2013-10-30 14:52 ` Victor Kaplansky
2013-10-30 15:39 ` Peter Zijlstra
2013-10-30 17:14 ` Victor Kaplansky
2013-10-30 17:44 ` Peter Zijlstra
2013-10-31 6:16 ` Paul E. McKenney
2013-11-01 13:12 ` Victor Kaplansky
2013-11-02 16:36 ` Paul E. McKenney
2013-11-02 17:26 ` Paul E. McKenney
2013-10-31 6:40 ` Paul E. McKenney
2013-11-01 14:25 ` Victor Kaplansky
2013-11-02 17:28 ` Paul E. McKenney
2013-11-01 14:56 ` Peter Zijlstra
2013-11-02 17:32 ` Paul E. McKenney
2013-11-03 14:40 ` Paul E. McKenney
2013-11-03 15:17 ` [RFC] arch: Introduce new TSO memory barrier smp_tmb() Peter Zijlstra
2013-11-03 18:08 ` Linus Torvalds
2013-11-03 20:01 ` Peter Zijlstra
2013-11-03 22:42 ` Paul E. McKenney
2013-11-03 23:34 ` Linus Torvalds
2013-11-04 10:51 ` Paul E. McKenney
2013-11-04 11:22 ` Peter Zijlstra
2013-11-04 16:27 ` Paul E. McKenney
2013-11-04 16:48 ` Peter Zijlstra
2013-11-04 19:11 ` Peter Zijlstra
2013-11-04 19:18 ` Peter Zijlstra
2013-11-04 20:54 ` Paul E. McKenney
2013-11-04 20:53 ` Paul E. McKenney
2013-11-05 14:05 ` Will Deacon
2013-11-05 14:49 ` Paul E. McKenney
2013-11-05 18:49 ` Peter Zijlstra
2013-11-06 11:00 ` Will Deacon
2013-11-06 12:39 ` Peter Zijlstra
2013-11-06 12:51 ` Geert Uytterhoeven
2013-11-06 13:57 ` Peter Zijlstra
2013-11-06 18:48 ` Paul E. McKenney
2013-11-06 19:42 ` Peter Zijlstra
2013-11-07 11:17 ` Will Deacon
2013-11-07 13:36 ` Peter Zijlstra
2013-11-07 23:50 ` Mathieu Desnoyers
2013-11-04 11:05 ` Will Deacon [this message]
2013-11-04 16:34 ` Paul E. McKenney
2013-11-03 20:59 ` Benjamin Herrenschmidt
2013-11-03 22:43 ` Paul E. McKenney
2013-11-03 17:07 ` perf events ring buffer memory barrier on powerpc Will Deacon
2013-11-03 22:47 ` Paul E. McKenney
2013-11-04 9:57 ` Will Deacon
2013-11-04 10:52 ` Paul E. McKenney
2013-11-01 16:11 ` Peter Zijlstra
2013-11-02 17:46 ` Paul E. McKenney
2013-11-01 16:18 ` Peter Zijlstra
2013-11-02 17:49 ` Paul E. McKenney
2013-10-30 13:28 ` Victor Kaplansky
2013-10-30 15:51 ` Peter Zijlstra
2013-10-30 18:29 ` Peter Zijlstra
2013-10-30 19:11 ` Peter Zijlstra
2013-10-31 4:33 ` Paul E. McKenney
2013-10-31 4:32 ` Paul E. McKenney
2013-10-31 9:04 ` Peter Zijlstra
2013-10-31 15:07 ` Paul E. McKenney
2013-10-31 15:19 ` Peter Zijlstra
2013-11-01 9:28 ` Paul E. McKenney
2013-11-01 10:30 ` Peter Zijlstra
2013-11-02 15:20 ` Paul E. McKenney
2013-11-04 9:07 ` Peter Zijlstra
2013-11-04 10:00 ` Paul E. McKenney
2013-10-31 9:59 ` Victor Kaplansky
2013-10-31 12:28 ` David Laight
2013-10-31 12:55 ` Victor Kaplansky
2013-10-31 15:25 ` Paul E. McKenney
2013-11-01 16:06 ` Victor Kaplansky
2013-11-01 16:25 ` David Laight
2013-11-01 16:30 ` Victor Kaplansky
2013-11-03 20:57 ` Benjamin Herrenschmidt
2013-11-02 15:46 ` Paul E. McKenney
2013-10-28 19:09 ` Oleg Nesterov
2013-10-29 14:06 ` [tip:perf/urgent] perf: Fix perf ring buffer memory ordering tip-bot for Peter Zijlstra
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