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

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Message-ID: <20170801121419.a365inyyk5hghb6w@hirez.programming.kicks-ass.net>
Date:   Tue, 1 Aug 2017 14:14:19 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Will Deacon <will.deacon@....com>, 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, 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.. :-)