Re: [RFC][PATCH 1/5] mm: Rework {set,clear,mm}_tlb_flush_pending()

From: Peter Zijlstra
Date: Tue Aug 01 2017 - 08:15:00 EST

On Tue, Aug 01, 2017 at 10:02:45PM +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2017-08-01 at 11:31 +0100, Will Deacon wrote:
> > Looks like that's what's currently relied upon:
> >
> > /* Clearing is done after a TLB flush, which also provides a barrier. */
> >
> > It also provides barrier semantics on arm/arm64. In reality, I suspect
> > all archs have to provide some order between set_pte_at and flush_tlb_range
> > which is sufficient to hold up clearing the flag. :/
> Hrm... not explicitely.
> Most archs (powerpc among them) have set_pte_at be just a dumb store,
> so the only barrier it has is the surrounding PTL.
> Now flush_tlb_range() I assume has some internal strong barriers but
> none of that is well defined or documented at all, so I suspect all
> bets are off.

Right.. but seeing how we're in fact relying on things here it might be
time to go figure this out and document bits.

*sigh*, I suppose its going to be me doing this.. :-)