Re: [RFC][PATCH 0/5] arch: atomic rework

[Torvald Riegel]

On Mon, 2014-02-17 at 14:14 -0800, Paul E. McKenney wrote:
> On Mon, Feb 17, 2014 at 09:39:54PM +0100, Richard Biener wrote:
> > On February 17, 2014 7:18:15 PM GMT+01:00, "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > >On Wed, Feb 12, 2014 at 07:12:05PM +0100, Peter Zijlstra wrote:
> > >> On Wed, Feb 12, 2014 at 09:42:09AM -0800, Paul E. McKenney wrote:
> > >> > You need volatile semantics to force the compiler to ignore any
> > >proofs
> > >> > it might otherwise attempt to construct.  Hence all the
> > >ACCESS_ONCE()
> > >> > calls in my email to Torvald.  (Hopefully I translated your example
> > >> > reasonably.)
> > >> 
> > >> My brain gave out for today; but it did appear to have the right
> > >> structure.
> > >
> > >I can relate.  ;-)
> > >
> > >> I would prefer it C11 would not require the volatile casts. It should
> > >> simply _never_ speculate with atomic writes, volatile or not.
> > >
> > >I agree with not needing volatiles to prevent speculated writes. 
> > >However,
> > >they will sometimes be needed to prevent excessive load/store
> > >combining.
> > >The compiler doesn't have the runtime feedback mechanisms that the
> > >hardware has, and thus will need help from the developer from time
> > >to time.
> > >
> > >Or maybe the Linux kernel simply waits to transition to C11 relaxed
> > >atomics
> > >until the compiler has learned to be sufficiently conservative in its
> > >load-store combining decisions.
> > 
> > Sounds backwards. Currently the compiler does nothing to the atomics. I'm sure we'll eventually add something. But if testing coverage is zero outside then surely things get worse, not better with time.
> 
> Perhaps we solve this chicken-and-egg problem by creating a test suite?

Perhaps.  The test suite might also be a good set of examples showing
which cases we expect to be optimized in a certain way, and which not.
I suppose the uses of (the equivalent) of atomics in the kernel would be
a good start.

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