linux-kernel - Re: [PATCH v5 06/21] perf script: Move find

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Message-ID: <ZykxD41c6gWQoIrQ@x1>
Date: Mon, 4 Nov 2024 17:39:43 -0300
From: Arnaldo Carvalho de Melo <acme@...nel.org>
To: Ian Rogers <irogers@...gle.com>
Cc: Namhyung Kim <namhyung@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Kan Liang <kan.liang@...ux.intel.com>,
	James Clark <james.clark@...aro.org>,
	Howard Chu <howardchu95@...il.com>,
	Athira Jajeev <atrajeev@...ux.vnet.ibm.com>,
	Michael Petlan <mpetlan@...hat.com>,
	Veronika Molnarova <vmolnaro@...hat.com>,
	Dapeng Mi <dapeng1.mi@...ux.intel.com>,
	Thomas Richter <tmricht@...ux.ibm.com>,
	Ilya Leoshkevich <iii@...ux.ibm.com>,
	Colin Ian King <colin.i.king@...il.com>,
	Weilin Wang <weilin.wang@...el.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>, linux-kernel@...r.kernel.org,
	linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v5 06/21] perf script: Move find_scripts to
 browser/scripts.c

On Mon, Nov 04, 2024 at 12:34:47PM -0800, Ian Rogers wrote:
> On Mon, Nov 4, 2024 at 11:47 AM Namhyung Kim <namhyung@...nel.org> wrote:
> >
> > On Thu, Oct 31, 2024 at 01:51:36PM -0700, Ian Rogers wrote:
> > > On Thu, Oct 31, 2024 at 12:18 PM Arnaldo Carvalho de Melo
> > > <acme@...nel.org> wrote:
> > > >
> > > > On Wed, Oct 30, 2024 at 06:42:37PM -0700, Ian Rogers wrote:
> > > > > The only use of find_scripts is in browser/scripts.c but the
> > > > > definition in builtin causes linking problems requiring a stub in
> > > > > python.c. Move the function to allow the stub to be removed.
> > > > >
> > > > > Rewrite the directory iteration to use openat so that large character
> > > > > arrays aren't needed. The arrays are warned about potential buffer
> > > > > overflows by GCC now that all the code exists in a single C file.
> > > >
> > > > Introducing is_directory_at() should be done as a prep patch, as the
> > > > rest of the patch below could end up being reverted after some other
> > > > patch used it, making the process more difficult.
> > > >
> > > > I mentioned cases like this in the past, so doing it again just for the
> > > > record.
> > >
> > > This is highlighted in the commit message:
> > > ```
> > > Rewrite the directory iteration to use openat so that large character
> > > arrays aren't needed. The arrays are warned about potential buffer
> > > overflows by GCC now that all the code exists in a single C file.
> > > ```
> > > so without the change the code wouldn't build. The new is_directory_at
> > > function is effectively 2 statements fstatat and S_ISDIR on the
> > > result, it is put next to is_directory for consistency but could have
> > > been a static function in the only C file to use it.
> > >
> > > For the record, patches introducing 2 line long functions can be
> > > excessively noisy, especially in a 21 patch series. There is always
> > > the declared but not used build error to worry about - here things
> > > couldn't just be simply moved due to triggering a different build
> > > error. Given the simplicity of the function here I made a decision not
> > > to split up the work - the commit message would likely be longer than
> > > the function. The work never intended to introduce is_directory_at but
> > > was forced into it through a desire not to disable compiler warnings.
> >
> > This patch does more than just moving the code which can be easy to miss
> > something in the middle.  I think you can move the code as is without
> > introducing build errors and then add new changes like using openat() on
> > top (you may separate the change out of this series).  I think it's
> > ok to have a small change if it clearly has different semantics.
> 
> If you are trying to bisect to find something that broke a build,
> having commits that knowingly break the build will cause the bisect to
> fail. The bisect will falsely fail on the known to be broken commit.

I'm not understanding, AFAIK nobody is advocating for breaking
bisection, just to first instroduce a function, then use it to avoid:

1) Introduce function foo() and use it for feature bar()
2) Somebody else uses function foo()
3) We find a justification to revert 1) but can't, since it will break
   2) so we need to add 4) that removes bar() from 1).

- Arnaldo

> It should be unacceptable to knowingly break the build in a commit for
> this reason.
> 
> Thanks,
> Ian