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

[Segher Boessenkool <segher@kernel.crashing.org>]

> 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.

There is no such thing as "volatile-qualified access" defined
anywhere; there only is the concept of a "volatile-qualified
*object*".

> 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).

There is a quite convincing argument that such an access _is_ an
access to a volatile object; see GCC PR21568 comment #9.  This
probably isn't the last word on the matter though...


Segher

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