linux-kernel - Re: cleanup: Make no_free_ptr() __must

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Message-ID: <fcc8a158-f6e4-8963-782f-ba04b47350b8@rasmusvillemoes.dk>
Date:   Tue, 15 Aug 2023 13:28:37 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        dan.j.williams@...el.com
Cc:     linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: cleanup: Make no_free_ptr() __must_check

On 15/08/2023 12.52, Peter Zijlstra wrote:
> 
> recent discussion brought about the realization that it makes sense for
> no_free_ptr() to have __must_check semantics in order to avoid leaking
> the resource.
> 

> +static inline __must_check void * __no_free_ptr(void **pp)
> +{ void *p = *pp; *pp = NULL; return p; }
> +
>  #define no_free_ptr(p) \
> -	({ __auto_type __ptr = (p); (p) = NULL; __ptr; })
> +	(({ void * __maybe_unused ___t = (p); }), \
> +	 ((typeof(p))__no_free_ptr((void **)&(p))))

So this does seem to work as advertised, but it could perhaps use some
comments. Because at first I read this as one big statement expression,
and I had this memory of a __must_check function call being the last
expression in such had no effect at all [1]. But this is actually a
comma expression.

Also, isn't it more complicated than necessary? Can we get rid of the
inner stmt expr and tmp var by just making it

  ((void) (p), ((typeof(p))__no_free_ptr((void **)&(p)))

which is more or less the whole reason comma expressions is a thing.

Rasmus

[1]
https://lore.kernel.org/lkml/6d190601-68f1-c086-97ac-2ee1c08f5a34@rasmusvillemoes.dk/