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

[Satyam Sharma <satyam@infradead.org>]

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).


Satyam
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