linux-kernel - Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

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Date:   Tue, 17 Jul 2018 12:40:09 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Alan Stern <stern@...land.harvard.edu>,
        andrea.parri@...rulasolutions.com,
        Will Deacon <will.deacon@....com>,
        Akira Yokosawa <akiyks@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nick Piggin <npiggin@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and
 remove it for ordinary release/acquire

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