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Date: Tue, 17 Mar 2015 17:37:56 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: linux-kernel@vger.kernel.org,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Nicholas Miell <nmiell@comcast.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>, Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        Lai Jiangshan <laijs@cn.fujitsu.com>,
        Stephen Hemminger <stephen@networkplumber.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        David Howells <dhowells@redhat.com>
Subject: Re: [RFC PATCH] sys_membarrier(): system/process-wide memory barrier
 (x86) (v12)
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On Tue, Mar 17, 2015 at 01:13:36PM +0000, Mathieu Desnoyers wrote:
> > Its basically: WMB + ACQUIRE, which theoretically can leak a read in,
> > but nobody sane _delays_ reads, you want to speculate reads, not
> > postpone.
> 
> If I believe the memory ordering table at
> https://en.wikipedia.org/wiki/Memory_ordering , there appears
> to be quite a few architectures that can reorder loads after loads,
> and loads after stores: Alpha, ARMv7, PA-RISC, SPARC RMO, x86 oostore
> and ia64. There may be subtle details that would allow us to
> do without the barriers in specific situations, but for that I'd
> very much like to hear what Paul has to say.

So I was starting to write that you can get load after load by one
speculating more than the other, but I suppose you can delay loads just
fine too.

Imagine getting a cache miss on a load, the OoO engine can then continue
execution until it hits a hard dependency, so you're effectively
delaying the load.

So yeah, if we want to be able to replace smp_rmb() with a
barrier+sys_membar() we need to promote the smp_mb__before_spinlock() to
smp_mb__after_unlock_lock() or so, that would only penalize PPC a bit.