Re: [PATCH 1/24] make atomic_read() behave consistently on alpha

[Geert Uytterhoeven <geert@linux-m68k.org>]

On Thu, 9 Aug 2007, Chris Snook wrote:
> Segher Boessenkool wrote:
> > > > The only safe way to get atomic accesses is to write
> > > > assembler code.  Are there any downsides to that?  I don't
> > > > see any.
> > > 
> > > The assumption that aligned word reads and writes are atomic, and that
> > > words are aligned unless explicitly packed otherwise, is endemic in the
> > > kernel.  No sane compiler violates this assumption.  It's true that we're
> > > not portable to insane compilers after this patch, but we never were in
> > > the first place.
> > 
> > You didn't answer my question: are there any downsides to using
> > explicit coded-in-assembler accesses for atomic accesses?  You
> > can handwave all you want that it should "just work" with
> > volatile accesses, but volatility != atomicity, volatile in C
> > is really badly defined, GCC never officially gave stronger
> > guarantees, and we have a bugzilla full of PRs to show what a
> > minefield it is.
> > 
> > So, why not use the well-defined alternative?
> 
> Because we don't need to, and it hurts performance.

It hurts performance by implementing 32-bit atomic reads in assembler?

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds