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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 17 Jul 2018 11:44:23 -0700
Message-ID: <CA+55aFwA=TmJJkKvskX3wL9Rs1j8xZJDJVghv=G0spYufK9kYg@mail.gmail.com>
Subject: Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and
 remove it for ordinary release/acquire
To:     Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc:     Michael Ellerman <mpe@ellerman.id.au>,
        Peter Zijlstra <peterz@infradead.org>,
        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>
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On Tue, Jul 17, 2018 at 11:31 AM Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
>
> 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.

That's only true when looking at isync in isolation.

Read the part I quoted. The AIX documentation implies that the
*sequence* of load-compare-conditional branch-isync is a memory
barrier, even if isync on its own is now.

So I'm just saying that

 (a) isync-on-lock is supposed to be much cheaper than sync-on-lock

 (b) the AIX documentation at least implies that isync-on-lock (when
used together the the whole locking sequence) is actually a memory
barrier

Now, admittedly the powerpc barrier instructions are unfathomable
crazy stuff, so who knows. But:

 (a) lwsync is a memory barrier for all the "easy" cases (ie
load->store, load->load, and store->load).

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

 (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!).

So a unlock/lock sequence where the unlock is using lwsync, and the
lock is using isync, should in fact be a full memory barrier (which is
the semantics we're looking for).

So doing performance testing on sync/lwsync (for lock/unlock
respectively) seems the wrong thing to do.  Please test the
isync/lwsync case instead.

Hmm? What am I missing?

              Linus