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

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Message-ID: <20211005144002.34008ea0@gandalf.local.home>
Date:   Tue, 5 Oct 2021 14:40:02 -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 20:28:54 +0200 (CEST)
Jan Engelhardt <jengelh@...i.de> wrote:

> 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*

Are you suggesting I should continue this exercise ;-)

-- Steve