linux-kernel - Re: [RFC][PATCH 1/5] mm: Rework {set,clear,mm}_tlb_flush

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Message-ID: <1501633061.2792.137.camel@kernel.crashing.org>
Date:   Wed, 02 Aug 2017 10:17:41 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
Cc:     torvalds@...ux-foundation.org, oleg@...hat.com,
        paulmck@...ux.vnet.ibm.com, mpe@...erman.id.au, npiggin@...il.com,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        stern@...land.harvard.edu, Mel Gorman <mgorman@...e.de>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [RFC][PATCH 1/5] mm: Rework {set,clear,mm}_tlb_flush_pending()

On Tue, 2017-08-01 at 18:48 +0200, Peter Zijlstra wrote:
> On Tue, Aug 01, 2017 at 05:44:14PM +0100, Will Deacon wrote:
> > On Tue, Aug 01, 2017 at 06:39:03PM +0200, Peter Zijlstra wrote:
> > > Still this is all rather unsatisfactory. Either we should define
> > > flush_tlb*() to imply a barrier when its not a no-op (sparc64/ppc-hash)
> > > or simply make clear_tlb_flush_pending() an smp_store_release().
> > > 
> > > I prefer the latter option.
> > > 
> > > Opinions?
> > 
> > I prefer the latter option too, since I'd like to relax the arm64 TLB
> > flushing to have weaker barriers for the local case. Granted, that doesn't
> > break the NUMA migration code, but it would make the barrier semantics of
> > the TLB invalidation routines even more subtle if we were to define them
> > generally.
> 
> Another 'fun' question, is smp_mb() strong enough to order against the
> TLB invalidate? Because we really want to clear this flag _after_.
> 
> PowerPC for example uses PTESYNC before the TBLIE, so does a SYNC after
> work? Ben?

I have no idea. But then our tlbie has a ptesync after too no ? And
afaik a ptesync is a superset of sync.

Cheers,
Ben.