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

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Date:   Tue, 5 Oct 2021 15:49:58 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     rostedt <rostedt@...dmis.org>
Cc:     Jan Engelhardt <jengelh@...i.de>,
        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 Oct 5, 2021, at 3:40 PM, rostedt rostedt@...dmis.org wrote:

> 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

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.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com