linux-kernel - Re: [PATCH v2 0/3] sysfs: constify bin_attribute argument of sysfs_bin_attr_simple

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Message-ID: <CAADnVQJMV-zRcDKftZ-MbKEJQ7XGmPteMYCS0Bm5siBEXUK=Fw@mail.gmail.com>
Date: Wed, 8 Jan 2025 09:45:45 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Thomas Weißschuh <linux@...ssschuh.net>
Cc: Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>, 
	Christophe Leroy <christophe.leroy@...roup.eu>, Naveen N Rao <naveen@...nel.org>, 
	Madhavan Srinivasan <maddy@...ux.ibm.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"Rafael J. Wysocki" <rafael@...nel.org>, Luis Chamberlain <mcgrof@...nel.org>, Petr Pavlu <petr.pavlu@...e.com>, 
	Sami Tolvanen <samitolvanen@...gle.com>, Daniel Gomez <da.gomez@...sung.com>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, ppc-dev <linuxppc-dev@...ts.ozlabs.org>, 
	LKML <linux-kernel@...r.kernel.org>, linux-modules@...r.kernel.org, 
	bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH v2 0/3] sysfs: constify bin_attribute argument of sysfs_bin_attr_simple_read()

On Tue, Dec 31, 2024 at 2:30 AM Thomas Weißschuh <linux@...ssschuh.net> wrote:
>
> On 2024-12-30 16:50:41-0800, Alexei Starovoitov wrote:
> > On Sat, Dec 28, 2024 at 12:43 AM Thomas Weißschuh <linux@...ssschuh.net> wrote:
> > >
> > > Most users use this function through the BIN_ATTR_SIMPLE* macros,
> > > they can handle the switch transparently.
> > >
> > > This series is meant to be merged through the driver core tree.
> >
> > hmm. why?
>
> Patch 1 changes the signature of sysfs_bin_attr_simple_read().
> Before patch 1 sysfs_bin_attr_simple_read() needs to be assigned to the
> callback member .read, after patch 1 it's .read_new.
> (Both callbacks work exactly the same, except for their signature,
> .read_new is only a transition mechanism and will go away again)
>
> > I'd rather take patches 2 and 3 into bpf-next to avoid
> > potential conflicts.
> > Patch 1 looks orthogonal and independent.
>
> If you pick up 2 and 3 through bpf-next you would need to adapt these
> assignments. As soon as both patch 1 and the modified 2 and 3 hit
> Linus' tree, the build would break due to mismatches function pointers.
> (Casting function pointers to avoid the mismatch will blow up with KCFI)

I see. All these steps to constify is frankly a mess.
You're wasting cpu and memory for this read vs read_new
when const is not much more than syntactic sugar in C.
You should have done one tree wide patch without doing this _new() hack.

Anyway, rant over. Carry patches 2,3. Hopefully they won't conflict.
But I don't want to see any constification patches in bpf land
that come with such pointless runtime penalty.