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Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE_ONCE when accessing page tables

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On Thu, Sep 28, 2017 at 09:45:35AM +0100, Will Deacon wrote:
> On Thu, Sep 28, 2017 at 10:38:01AM +0200, Peter Zijlstra wrote:
> > On Wed, Sep 27, 2017 at 04:49:28PM +0100, Will Deacon wrote:
> > > In many cases, page tables can be accessed concurrently by either another
> > > CPU (due to things like fast gup) or by the hardware page table walker
> > > itself, which may set access/dirty bits. In such cases, it is important
> > > to use READ_ONCE/WRITE_ONCE when accessing page table entries so that
> > > entries cannot be torn, merged or subject to apparent loss of coherence.
> > 
> > In fact, we should use lockless_dereference() for many of them. Yes
> > Alpha is the only one that cares about the difference between that and
> > READ_ONCE() and they do have the extra barrier, but if we're going to do
> > this, we might as well do it 'right' :-)
> 
> I know this sounds daft, but I think one of the big reasons why
> lockless_dereference() doesn't get an awful lot of use is because it's
> such a mouthful! Why don't we just move the smp_read_barrier_depends()
> into READ_ONCE? Would anybody actually care about the potential impact on
> Alpha (which, frankly, is treading on thin ice given the low adoption of
> lockless_dereference())?

This is my cue to ask my usual question...  ;-)

Are people still running mainline kernels on Alpha?  (Added Alpha folks.)

As always, if anyone is, we must continue to support Alpha, but sounds
like time to check again.

							Thanx, Paul

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[RFC PATCH 0/2] Missing READ_ONCE in core and arch-specific pgtable code leading to crashes Will Deacon <will.deacon@arm.com> - 2017-09-27 18:00 +0200
  Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE_ONCE when  accessing page tables Peter Zijlstra <peterz@infradead.org> - 2017-09-28 10:40 +0200
    Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE_ONCE when  accessing page tables Will Deacon <will.deacon@arm.com> - 2017-09-28 10:50 +0200
      Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE_ONCE when  accessing page tables "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-09-28 17:50 +0200
        Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE_ONCE when  accessing page tables Will Deacon <will.deacon@arm.com> - 2017-09-28 17:50 +0200
          Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE_ONCE when  accessing page tables "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-09-28 18:10 +0200
        Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE_ONCE when  accessing page tables Michael Cree <mcree@orcon.net.nz> - 2017-09-28 21:40 +0200
          Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE_ONCE when  accessing page tables "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-09-29 03:00 +0200
            Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE_ONCE when  accessing page tables Will Deacon <will.deacon@arm.com> - 2017-09-29 11:10 +0200
              Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE_ONCE when  accessing page tables Will Deacon <will.deacon@arm.com> - 2017-09-29 18:40 +0200
              Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE_ONCE when  accessing page tables "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-09-29 18:40 +0200
  Re: [RFC PATCH 0/2] Missing READ_ONCE in core and arch-specific  pgtable code leading to crashes Richard Ruigrok <rruigrok@codeaurora.org> - 2017-09-28 19:40 +0200
  Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE_ONCE when  accessing page tables Timur Tabi <timur@codeaurora.org> - 2017-09-28 21:20 +0200
  Re: [RFC PATCH 0/2] Missing READ_ONCE in core and arch-specific  pgtable code leading to crashes Jon Masters <jcm@redhat.com> - 2017-09-28 21:40 +0200
    Re: [RFC PATCH 0/2] Missing READ_ONCE in core and arch-specific  pgtable code leading to crashes Will Deacon <will.deacon@arm.com> - 2017-09-29 11:00 +0200

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