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

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Date:   Tue, 5 Oct 2021 15:40:29 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Jan Engelhardt <jengelh@...i.de>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Paul <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>,
        "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>, rcu <rcu@...r.kernel.org>,
        netfilter-devel <netfilter-devel@...r.kernel.org>,
        coreteam <coreteam@...filter.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [RFC][PATCH] rcu: Use typeof(p) instead of typeof(*p) *

On Tue, 5 Oct 2021 21:06:36 +0200 (CEST)
Jan Engelhardt <jengelh@...i.de> 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

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