linux-kernel - Re: Clarifying confusion of our variable placement rules caused by cleanup.h

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Message-ID: <CAHk-=wjD1r5dr_b7gpuPdTXGdXAtaM6wHBSJyzi-Zfw3-cqR6w@mail.gmail.com>
Date: Tue, 18 Nov 2025 12:30:07 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: James Bottomley <James.Bottomley@...senpartnership.com>
Cc: Steven Rostedt <rostedt@...dmis.org>, Bart Van Assche <bvanassche@....org>, ksummit@...ts.linux.dev, 
	Dan Williams <dan.j.williams@...el.com>, linux-kernel <linux-kernel@...r.kernel.org>, 
	Dan Carpenter <dan.carpenter@...aro.org>
Subject: Re: Clarifying confusion of our variable placement rules caused by cleanup.h

On Tue, 18 Nov 2025 at 12:21, James Bottomley
<James.Bottomley@...senpartnership.com> wrote:
>
> On Tue, 2025-11-18 at 14:17 -0500, Steven Rostedt wrote:
> > I think the code could also be better optimized? I haven't run an
> > objcopy to confirm but now early exits do not require calling the
> > __free() function on NULL pointers.
>
> Yes, I can confirm that (at least from reading the docs not actually
> from disassembling the code).

Actually, I _have_ been disassembling some of that code, and most of
the time the compiler is actually good at eliding those things and not
calling kfree() with a NULL pointer.

Now, the reason for that is actually that we spent some effort on this
in <linux/cleanup.h> (and by "we" I mean mostly PeterZ & co with me
being involved in the discussions).

So you'll see those destructor functions being inline functions with
things like that

   DEFINE_FREE(kfree, void *, if (_T) kfree(_T))

where that "if (_T)" being integral to having the compiler able to see
inline that "oh, it's statically NULL at this stage, I don't need to
call any external function".

But yes, sometimes having the declaration later can simplify this all
for the compiler too. But the *primary* thing should be about making
the code itself legible and maintainable to humans.

          Linus