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

On Tue, 5 Oct 2021 21:06:36 +0200 (CEST)
Jan Engelhardt [off-list ref] wrote:

On Tuesday 2021-10-05 20:40, Steven Rostedt wrote:

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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

struct trace_pid_list {
	raw_spinlock_t			lock;
	struct irq_work			refill_irqwork;
	union upper_chunk		*upper[UPPER1_SIZE]; // 1 or 2K in size
	union upper_chunk		*upper_list;
	union lower_chunk		*lower_list;
	int				free_upper_chunks;
	int				free_lower_chunks;
};

I can still abstract out the unions, but this means I need to also pull out
the define of "UPPER1_SIZE". Not to mention, I need to make sure irq_work
and spin locks are defined.

Another approach is to have it return:

struct trace_pid_list {
	unsigned long		ignore;
};

Rename the above struct trace_pid_list to struct trace_pid_internal.

And internally have:

union trace_pid_data {
	struct trace_pid_list		external;
	struct trace_pid_internal	internal;
};

Then use the internal version within the C file that modifies it, and just
return a pointer to the external part.

That should follow the "C standard".

-- Steve