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 11:49:41 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Peter Zijlstra <peterz@...radead.org>,
        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 11:44 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
>  (a) lwsync is a memory barrier for all the "easy" cases (ie
> load->store, load->load, and store->load).

That last one should have been "store->store", of course.

So 'lwsync' gives smp_rmb(), smp_wmb(), and smp_load_acquire()
semantics (which are the usual "no barrier needed at all" suspects for
things like x86).

What lwsync lacks is store->load ordering. So:

>  (b) lwsync is *not* a memory barrier for the store->load case.

BUT, this is where isync comes in:

>  (c) isync *is* (when in that *sequence*) a memory barrier for a
> store->load case (and has to be: loads inside a spinlocked region MUST
> NOT be done earlier than stores outside of it!).

which is why I think that a spinlock implementation that uses isync
would give us the semantics we want, without the use of the crazy
expensive 'sync' that Michael tested (and which apparently gets
horrible 10% scheduler performance regressions at least on some
powerpc CPU's).

            Linus