Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 17 Jul 2018 12:40:09 -0700
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Alan Stern <stern@rowland.harvard.edu>,
        andrea.parri@amarulasolutions.com,
        Will Deacon <will.deacon@arm.com>,
        Akira Yokosawa <akiyks@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Nick Piggin <npiggin@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and
 remove it for ordinary release/acquire
Reply-To: paulmck@linux.vnet.ibm.com
References: <CA+55aFxwez8zSa2f2GpcaAkFX27Vt-s0nWdd0mZ6yGM8ipCR0A@mail.gmail.com>
 <20180713110851.GY2494@hirez.programming.kicks-ass.net>
 <87tvp3xonl.fsf@concordia.ellerman.id.au>
 <20180713164239.GZ2494@hirez.programming.kicks-ass.net>
 <87601fz1kc.fsf@concordia.ellerman.id.au>
 <CA+55aFzjcKv40hT_aD4jsdERi+a1pLU2SWPTwvyT2PDnoOmSpQ@mail.gmail.com>
 <87va9dyl8y.fsf@concordia.ellerman.id.au>
 <CA+55aFyFLzH71Tke858x0cjCiRO48vc1W9oVmVkO43rtjezqOg@mail.gmail.com>
 <20180717183341.GQ12945@linux.vnet.ibm.com>
 <20180717184255.GM2494@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20180717184255.GM2494@hirez.programming.kicks-ass.net>
User-Agent: Mutt/1.5.21 (2010-09-15)
Message-Id: <20180717194009.GT12945@linux.vnet.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, Jul 17, 2018 at 08:42:55PM +0200, Peter Zijlstra wrote:
> On Tue, Jul 17, 2018 at 11:33:41AM -0700, Paul E. McKenney wrote:
> > On Tue, Jul 17, 2018 at 09:19:15AM -0700, Linus Torvalds wrote:
> > > 
> > > In particular, I find:
> > > 
> > >   "isync is not a memory barrier instruction, but the
> > > load-compare-conditional branch-isync sequence can provide this
> > > ordering property"
> > > 
> > > so why are you doing "sync/lwsync", when it sounds like "isync/lwsync"
> > > (for lock/unlock) is the right thing and would already give memory
> > > barrier semantics?
> > 
> > The PowerPC guys will correct me if I miss something here...
> > 
> > The isync provides ordering roughly similar to lwsync, but nowhere near
> > as strong as sync, and it is sync that would be needed to cause lock
> > acquisition to provide full ordering.  The reason for using lwsync instead
> > of isync is that the former proved to be faster on recent hardware.
> > The reason that the kernel still has the ability to instead generate
> > isync instructions is that some older PowerPC hardware does not provide
> > the lwsync instruction.  If the hardware does support lwsync, the isync
> > instructions are overwritten with lwsync at boot time.
> 
> Isn't ISYNC the instruction-sync pipeline flush instruction? That is
> used as an smp_rmb() here to, together with the control dependency from the
> LL/SC, to form a LOAD->{LOAD,STORE} (aka LOAD-ACQUIRE) ordering?

That is the one!

> Where LWSYNC provides a TSO like ordering and SYNC provides a full
> transitive barrier aka. smp_mb() (althgouh I think it is strictly
> stronger than smp_mb() since it also implies completion, which smp_mb()
> does not).
> 
> And since both LL/SC-CTRL + ISYNC / LWSYNC are strictly CPU local, they
> cannot be used to create RCsc ordering.

Completely agreed, there needs to be a sync instruction in either the
lock or the unlock to get RCsc ordering.  Neither lwsync nor isync
can provide the needed ordering.

							Thanx, Paul