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

From: Frederic Weisbecker <frederic@kernel.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>, Jiri Olsa <jolsa@redhat.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Yoshinori Sato <ysato@users.sourceforge.jp>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Chris Zankel <chris@zankel.net>,
	Paul Mackerras <paulus@samba.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Will Deacon <will.deacon@arm.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Rich Felker <dalias@libc.org>, Ingo Molnar <mingo@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Max Filippov <jcmvbkbc@gmail.com>
Subject: Re: [PATCH 8/9] perf/breakpoint: Split breakpoint "check" and "commit"
Date: Sat, 19 May 2018 04:42:37 +0200	[thread overview]
Message-ID: <20180519024236.GA4260@lerouge> (raw)
In-Reply-To: <99DA1A94-6AF6-4B9A-858A-7018D58CC38E@amacapital.net>

On Tue, May 15, 2018 at 09:58:03PM -0700, Andy Lutomirski wrote:
> 
> 
> > On May 15, 2018, at 8:11 PM, Frederic Weisbecker <frederic@kernel.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

Thanks, that works well!