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

[Satyam Sharma <satyam@infradead.org>]

On Thu, 16 Aug 2007, Satyam Sharma wrote:

> Hi Bill,
> 
> 
> On Wed, 15 Aug 2007, Bill Fink wrote:
> 
> > On Wed, 15 Aug 2007, Satyam Sharma wrote:
> > 
> > > (C)
> > > $ cat tp3.c
> > > int a;
> > > 
> > > void func(void)
> > > {
> > > 	*(volatile int *)&a = 10;
> > > 	*(volatile int *)&a = 20;
> > > }
> > > $ gcc -Os -S tp3.c
> > > $ cat tp3.s
> > > ...
> > > movl    $10, a
> > > movl    $20, a
> > > ...
> > 
> > I'm curious about one minor tangential point.  Why, instead of:
> > 
> > 	b = *(volatile int *)&a;
> > 
> > why can't this just be expressed as:
> > 
> > 	b = (volatile int)a;
> > 
> > Isn't it the contents of a that's volatile, i.e. it's value can change
> > invisibly to the compiler, and that's why you want to force a read from
> > memory?  Why do you need the "*(volatile int *)&" construct?
> 
> "b = (volatile int)a;" doesn't help us because a cast to a qualified type
> has the same effect as a cast to an unqualified version of that type, as
> mentioned in 6.5.4:4 (footnote 86) of the standard. Note that "volatile"
> is a type-qualifier, not a type itself, so a cast of the _object_ itself
> to a qualified-type i.e. (volatile int) would not make the access itself
> volatile-qualified.
> 
> To serve our purposes, it is necessary for us to take the address of this
> (non-volatile) object, cast the resulting _pointer_ to the corresponding
> volatile-qualified pointer-type, and then dereference it. This makes that
> particular _access_ be volatile-qualified, without the object itself being
> such. Also note that the (dereferenced) result is also a valid lvalue and
> hence can be used in "*(volatile int *)&a = b;" kind of construction
> (which we use for the atomic_set case).

Here, I should obviously admit that the semantics of *(volatile int *)&
aren't any neater or well-defined in the _language standard_ at all. The
standard does say (verbatim) "precisely what constitutes as access to
object of volatile-qualified type is implementation-defined", but GCC
does help us out here by doing the right thing. Accessing the non-volatile
object there using the volatile-qualified pointer-type cast makes GCC
treat the object stored at that memory address itself as if it were a 
volatile object, thus making the access end up having what we're calling
as "volatility" semantics here.

Honestly, given such confusion, and the propensity of the "volatile"
type-qualifier keyword to be ill-defined (or at least poorly understood,
often inconsistently implemented), I'd (again) express my opinion that it
would be best to avoid its usage, given other alternatives do exist.