Re: [PATCH 3/4] x86,asm: Re-work smp_store_mb()

[Davidlohr Bueso]

On Wed, 28 Oct 2015, Linus Torvalds wrote:

> On Wed, Oct 28, 2015 at 4:53 AM, Davidlohr Bueso <dave@xxxxxxxxxxxx> wrote:
> > Note that this might affect callers that could/would rely on the
> > atomicity semantics, but there are no guarantees of that for
> > smp_store_mb() mentioned anywhere, plus most archs use this anyway.
> > Thus we continue to be consistent with the memory-barriers.txt file,
> > and more importantly, maintain the semantics of the smp_ nature.
>
> So I dislike this patch, mostly because it now makes it obvious that
> smp_store_mb() seems to be totally pointless. Every single
> implementation is now apparently WRITE_ONCE+smp_mb(), and there are
> what, five users of it, so why not then open-code it?

So after having gone through pretty much all of smp_store_mb code, this
is a feeling I also share. However I justified its existence (as opposed
to dropping the call, updating all the callers/documenting the barriers
etc.) to at least encapsulate the store+mb logic, which apparently is a
pattern somewhat needed(?). Also, the name is obviously exactly what its
name implies.

But I have no strong preference either way. Now, if we should keep
smp_store_mb(), it should probably be made generic, instead of having
each arch define it.

> But more importantly, is the "WRITE_ONCE()" even necessary? If there
> are no atomicity guarantees, then why bother with WRTE_ONCE() either?

Agreed. Hmm, this was introduced by ab3f02fc237 (locking/arch: Add WRITE_ONCE()
to set_mb()), back when atomicity aspects were not clear yet.

> So with this patch, the whole thing becomes pointless, I feel. (Ok, so
> it may have been pointless before too, but at least before this patch
> it generated special code, now it doesn't). So why carry it along at
> all?

Ok, unless others are strongly against it, I'll send a series to drop the
call altogether.

Thanks,
Davidlohr
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