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

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Message-ID: <826o327o-3r46-3oop-r430-8qr0ssp537o3@vanv.qr>
Date:   Tue, 5 Oct 2021 20:28:54 +0200 (CEST)
From:   Jan Engelhardt <jengelh@...i.de>
To:     rostedt <rostedt@...dmis.org>
cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        rostedt <rostedt@...dmis.org>,
        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 Tuesday 2021-10-05 20:06, Mathieu Desnoyers wrote:
>> instead of just "typeof(p)", to force the decay to a pointer.
>
>If the type of @p is an integer, (p) + 0 is still valid, so it will not
>prevent users from passing an integer type as argument, which is what
>the current implementation prevents.
>
>Also, AFAIU, the compiler wants to know the sizeof(p) in order to evaluate
>(p + 0). Steven's goal is to hide the structure declaration, so that would
>not work either.

>>>> 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*