linux-kernel - Re: [PATCH 2/2]: atomic_t: Remove volatile from atomic

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Date:	Fri, 21 May 2010 15:27:46 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	David Miller <davem@...emloft.net>, torvalds@...ux-foundation.org,
	anton@...ba.org, akpm@...ux-foundation.org, willy@...ux.intel.com,
	benh@...nel.crashing.org, paulus@...ba.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2]: atomic_t: Remove volatile from atomic_t definition

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@...ux.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?


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