linux-kernel - Re: [PATCH 8/9] perf/breakpoint: Split breakpoint "check" and "commit"

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Date:   Tue, 15 May 2018 21:58:03 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Chris Zankel <chris@...kel.net>,
        Paul Mackerras <paulus@...ba.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will.deacon@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Rich Felker <dalias@...c.org>, Ingo Molnar <mingo@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Max Filippov <jcmvbkbc@...il.com>
Subject: Re: [PATCH 8/9] perf/breakpoint: Split breakpoint "check" and "commit"



> On May 15, 2018, at 8:11 PM, Frederic Weisbecker <frederic@...nel.org> wrote:
> 
>> On Wed, May 09, 2018 at 11:17:03AM +0200, Peter Zijlstra wrote:
>>> On Sun, May 06, 2018 at 09:19:54PM +0200, Frederic Weisbecker wrote:
>>> arch/arm/include/asm/hw_breakpoint.h     |  5 ++++-
>>> arch/arm/kernel/hw_breakpoint.c          | 22 +++-------------------
>>> arch/arm64/include/asm/hw_breakpoint.h   |  5 ++++-
>>> arch/arm64/kernel/hw_breakpoint.c        | 22 +++-------------------
>>> arch/powerpc/include/asm/hw_breakpoint.h |  5 ++++-
>>> arch/powerpc/kernel/hw_breakpoint.c      | 22 +++-------------------
>>> arch/sh/include/asm/hw_breakpoint.h |  5 ++++-
>>> arch/sh/kernel/hw_breakpoint.c           | 22 +++-------------------
>>> arch/x86/include/asm/hw_breakpoint.h     |  5 ++++-
>>> arch/x86/kernel/hw_breakpoint.c          | 23 +++--------------------
>>> arch/xtensa/include/asm/hw_breakpoint.h  |  5 ++++-
>>> arch/xtensa/kernel/hw_breakpoint.c       | 22 +++-------------------
>> 
>> Because of those ^,
>> 
>>> kernel/events/hw_breakpoint.c            | 11 ++++++-----
>> 
>> would it not make sense to have a prelimenary patch doing something
>> like:
>> 
>> __weak int hw_breakpoint_arch_check(struct perf_event *bp)
>> {
>>    return arch_validate_hwbkpt_settings(bp);
>> }
> 
> So eventually I fear I can't do that, due to linking order.
> 
> Say I convert x86 to implement hw_breakpoint_arch_check(), so I
> remove arch_validate_hwbkpt_settings(). On build time, the weak version
> is still compiled and can't find a declaration for arch_validate_hwbkpt_settings().
> 
> I tried to keep the declaration while the definition has been removed but
> it seems the weak version is linked first before it gets later replaced by
> the overriden arch version. So I get a build error.
> 
> I could keep arch_validate_hwbkpt_settings() around on all archs and remove it in
> the end with the weak version but that would defeat the purpose of removing
> the mid-state in the current patch.

How about just not using weak functions?  Weak functions have annoying issues like this, and they have trouble generating good code. I much prefer the pattern:

in arch header:
extern void arch_func(whatever);
#define arch_func arch_func

in generic header:
#ifndef arch_func
static inline void arch_func(whatever) ...
#endif