Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

[Segher Boessenkool]

> > > > atomic_dec() writes
> > > > to memory, so it _does_ have "volatile semantics", implicitly, as
> > > > long as the compiler cannot optimise the atomic variable away
> > > > completely -- any store counts as a side effect.
> > >
> > > I don't think an atomic_dec() implemented as an inline "asm volatile"
> > > or one that uses a "forget" macro would have the same re-ordering
> > > guarantees as an atomic_dec() that uses a volatile access cast.
> >
> > The "asm volatile" implementation does have exactly the same
> > reordering guarantees as the "volatile cast" thing,
>
> I don't think so.

"asm volatile" creates a side effect.  Side effects aren't
allowed to be reordered wrt sequence points.  This is exactly
the same reason as why "volatile accesses" cannot be reordered.

> > if that is
> > implemented by GCC in the "obvious" way.  Even a "plain" asm()
> > will do the same.
>
> Read the relevant GCC documentation.

I did, yes.

> [ of course, if the (latest) GCC documentation is *yet again*
>   wrong, then alright, not much I can do about it, is there. ]

There was (and is) nothing wrong about the "+m" documentation, if
that is what you are talking about.  It could be extended now, to
allow "+m" -- but that takes more than just "fixing" the documentation.


Segher

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