Re: asm-generic: Disallow no-op mb() for SMP systems

[Will Deacon <will.deacon@arm.com>]

On Thu, Feb 01, 2018 at 02:29:09PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 01, 2018 at 09:27:50PM +0900, Stafford Horne wrote:
> > I tried to clarify some of this in the spec v1.2 [0] which help formalize some of
> > the techniques we used for the SMP implementation.  Its probably not perfect,
> > but I added a section "10. Multicore support" and tried to clarify some things
> > in section 7 on Atomicity.  But it seems I dont cover exactly what are are
> > mentioning here.  In general:
> > 
> >   1 Secondary cores have memory snooping enabled meaning that any write to a
> >     cached address will cause the cache line to be invalidated.
> >   2 l.swa (store atomic word) implies a store buffer flush.
> 
> What about l.lwa? Can that observe 'old' values, or rather, miss values
> stuck in a remote store buffer?
> 
> This will then cause the first l.swa to fail, which, per the above,
> would then sync things up? Which means you get that one extra
> merry-go-round.

That's ok from a correctness perspective, though, as long as store buffers
are guaranteed to drain.

> >   3 l.msync is used to flush the store buffer
> > 
> > Also, during the IPI controller review [1] Marc Z asked many similar questions.
> > I believe he was ok in the end.
> > 
> > Anyway,
> > Thanks for thanks for spotting the issue here.  For some reason I remember we
> > did have an l.msync for our mb().  Let me think about and test out this patch
> > (and the fix to actually define mb) to see if anything comes up.
> > 
> > Also, I haven't seen any implementations that use WOM.  Stefan might know better.
> 
> So if the strong model has a store buffer, as I think the above says,
> then it is _NOT_ correct for l.msync to be treated as a NOP, it _must_
> flush the store buffer.
> 
> At which point I think your 'strong' model is basically TSO. So it would
> be very good to get that spelled out somewhere.

Agreed, especially as a software person reading the spec may end up thinking
that they don't need to use l.msync if they never set WOM. At least, I read
it this way despite knowing that it's probably not what you intended.

Will