Re: [RFC][PATCH] rcu: Use typeof(p) instead of typeof(*p) *

[Steven Rostedt]

On Tue, 5 Oct 2021 15:49:58 -0400 (EDT)
Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:

> ----- On Oct 5, 2021, at 3:40 PM, rostedt rostedt@xxxxxxxxxxx wrote:
> 
> > On Tue, 5 Oct 2021 21:06:36 +0200 (CEST)
> > Jan Engelhardt <jengelh@xxxxxxx> wrote:
> >   
> >> On Tuesday 2021-10-05 20:40, Steven Rostedt wrote:  
> >> >>     
> >> >> >>>> typeof(*p) *________p1 = (typeof(*p) *__force)READ_ONCE(p);  
> >> >> 
> >> >> #define static_cast(type, expr) ((struct { type x; }){(expr)}.x)
> >> >> typeof(p) p1 = (typeof(p) __force)static_cast(void *, READ_ONCE(p));
> >> >> 
> >> >> Let the name not fool you; it's absolutely _not_ the same as C++'s
> >> >> static_cast, but still: it does emit a warning when you do pass an
> >> >> integer, which is better than no warning at all in that case.
> >> >> 
> >> >>  *flies away*  
> >> >
> >> >Are you suggesting I should continue this exercise ;-)  
> >> 
> >> “After all, why not?”
> >> 
> >> typeof(p) p1 = (typeof(p) __force)READ_ONCE(p) +
> >>                BUILD_BUG_ON_EXPR(__builtin_classify_type(p) != 5);  
> > 
> > I may try it, because exposing the structure I want to hide, is pulling out
> > a lot of other crap with it :-p  
> 
> I like the static_cast() approach above. It is neat way to validate that the
> argument is a pointer without need to dereference the pointer.
> 
> I would also be open to consider this trick for liburcu's userspace API.
> 
> About the other proposed solution based on __builtin_classify_type, I am
> reluctant to use something designed specifically for varargs in a context
> where they are not used.
> 

Unfortunately, it doesn't solve the Debian gcc 10 compiler failing when
passing the function name instead of a pointer to the function in
RCU_INIT_POINTER()  That alone makes me feel like I shouldn't touch that
macro :-(

And who knows what other version of gcc will fail on passing the address :-p

-- Steve