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

[Paul E. McKenney]

On Fri, Sep 29, 2017 at 10:08:43AM +0100, Will Deacon wrote:
> On Thu, Sep 28, 2017 at 05:58:30PM -0700, Paul E. McKenney wrote:
> > On Fri, Sep 29, 2017 at 07:59:09AM +1300, Michael Cree wrote:
> > > On Thu, Sep 28, 2017 at 08:43:54AM -0700, Paul E. McKenney wrote:
> > > > 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.)
> > > 
> > > Yes.  I run two Alpha build daemons that build the unofficial
> > > debian-alpha port.  Debian popcon reports nine machines running
> > > Alpha, which are likely to be running the 4.12.y kernel which
> > > is currently in debian-alpha, (and presumably soon to be 4.13.y
> > > which is now built on Alpha in experimental).
> > 
> > I salute your dedication to Alpha!  ;-)
> 
> Ok, but where does that leave us wrt my initial proposal of moving
> smp_read_barrier_depends() into READ_ONCE and getting rid of
> lockless_dereference?
> 
> Michael (or anybody else running mainline on SMP Alpha) -- would you be
> able to give the diff below a spin and see whether there's a measurable
> performance impact?

This will be a sensitive test.  The smp_read_barrier_depends() can be
removed from lockless_dereference().  Without this removal Alpha will
get two memory barriers from rcu_dereference() and friends.

							Thanx, Paul

> Cheers,
> 
> Will
> 
> --->8
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index e95a2631e545..0ce21e25492a 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -340,6 +340,7 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
>  		__read_once_size(&(x), __u.__c, sizeof(x));		\
>  	else								\
>  		__read_once_size_nocheck(&(x), __u.__c, sizeof(x));	\
> +	smp_read_barrier_depends(); /* Enforce dependency ordering from x */ \
>  	__u.__val;							\
>  })
>  #define READ_ONCE(x) __READ_ONCE(x, 1)
> 

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