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Date:   Tue, 5 Oct 2021 15:49:58 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     rostedt <rostedt@goodmis.org>
Cc:     Jan Engelhardt <jengelh@inai.de>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Paul <paulmck@linux.vnet.ibm.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        "Joel Fernandes, Google" <joel@joelfernandes.org>,
        Pablo Neira Ayuso <pablo@netfilter.org>,
        Jozsef Kadlecsik <kadlec@netfilter.org>,
        Florian Westphal <fw@strlen.de>,
        "David S. Miller" <davem@davemloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>,
        Jakub Kicinski <kuba@kernel.org>, rcu <rcu@vger.kernel.org>,
        netfilter-devel <netfilter-devel@vger.kernel.org>,
        coreteam <coreteam@netfilter.org>,
        netdev <netdev@vger.kernel.org>
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Subject: Re: [RFC][PATCH] rcu: Use typeof(p) instead of typeof(*p) *
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Thread-Topic: Use typeof(p) instead of typeof(*p) *
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----- On Oct 5, 2021, at 3:40 PM, rostedt rostedt@goodmis.org wrote:

> On Tue, 5 Oct 2021 21:06:36 +0200 (CEST)
> Jan Engelhardt <jengelh@inai.de> wrote:
>=20
>> On Tuesday 2021-10-05 20:40, Steven Rostedt wrote:
>> >>  =20
>> >> >>>> typeof(*p) *________p1 =3D (typeof(*p) *__force)READ_ONCE(p);
>> >>=20
>> >> #define static_cast(type, expr) ((struct { type x; }){(expr)}.x)
>> >> typeof(p) p1 =3D (typeof(p) __force)static_cast(void *, READ_ONCE(p))=
;
>> >>=20
>> >> 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.
>> >>=20
>> >>  *flies away*
>> >
>> >Are you suggesting I should continue this exercise ;-)
>>=20
>> =E2=80=9CAfter all, why not?=E2=80=9D
>>=20
>> typeof(p) p1 =3D (typeof(p) __force)READ_ONCE(p) +
>>                BUILD_BUG_ON_EXPR(__builtin_classify_type(p) !=3D 5);
>=20
> I may try it, because exposing the structure I want to hide, is pulling o=
ut
> a lot of other crap with it :-p

I like the static_cast() approach above. It is neat way to validate that th=
e
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

--=20
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com