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

[Andy Lutomirski]

> On May 15, 2018, at 8:11 PM, Frederic Weisbecker <frederic@xxxxxxxxxx> 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