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

Linus Torvalds
2018-07-17T18:49:56Z

On Tue, Jul 17, 2018 at 11:44 AM Linus Torvalds
<torvalds@linux-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