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

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Message-ID: <2025010914-gangly-trodden-aa96@gregkh>
Date: Thu, 9 Jan 2025 08:56:37 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Thomas Weißschuh <linux@...ssschuh.net>,
	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>,
	"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 Wed, Jan 08, 2025 at 09:45:45AM -0800, Alexei Starovoitov wrote:
> 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.

The "pointless" penalty will go away once we convert all instances, and
really, it's just one pointer check, sysfs files should NOT be a hot
path for anything real, and one more pointer check should be cached and
not measurable compared to the real logic behind the binary data coming
from the hardware/kernel, right?

sysfs is NOT tuned for speed at all, so adding more checks like this
should be fine.

thanks,

greg k-h