Re: [PATCH 2/2]: atomic_t: Remove volatile from atomic_t definition

Nick Piggin
2010-05-21T05:27:53Z

On Wed, May 19, 2010 at 03:50:46PM -0700, Paul E. McKenney wrote:
> On Wed, May 19, 2010 at 12:54:49PM -0700, David Miller wrote:
> > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > Date: Wed, 19 May 2010 08:01:32 -0700
> >
> > > On Wed, May 19, 2010 at 11:03:27PM +1000, Nick Piggin wrote:
> > >> For atomic_read it shouldn't matter unless gcc is *really* bad at it.
> > >> Ah, for atomic_read, the required semantic is surely ACCESS_ONCE, so
> > >> that's where the volatile is needed? (maybe it would be clearer to
> > >> explicitly use ACCESS_ONCE?)
> > >
> > > Explicit use of ACCESS_ONCE() where needed makes a lot of sense to me,
> > > and allows better code to be generated for initialization and cleanup
> > > code where no other task has access to the atomic_t.
> >
> > I agree and I want to see this too, but I think with the tree the size
> > that it is we have to work backwards at this point.
> >
> > Existing behavior by default, and optimized cases get tagged by using
> > a new interface (atomic_read_light(), test_bit{,s}_light(), etc.)
>
> Fair enough!

Hmm, I'm missing something. David, back up a second, as far as I can see,
with Anton's patches, atomic_read() *is* effectively just ACCESS_ONCE()
now. Linus pointed out that header tangle is the reason not to just use
the macro.

Am I wrong, or is it that ACCESS_ONCE has a more relaxed semantic *in
theory* that allows a future more aggressive implementation if the
compiler supports it?

do {
done = ACCESS_ONCE(blah);
} while (!done);

Is this (in theory) allowed to be turned into a branch into an infinite
loop? Wheras the volatile deref would require it be reloaded each time?