linux-kernel - Re: [PATCH] powerpc: Mark variable `cpumsr` as unused

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Date:   Thu, 8 Nov 2018 11:59:11 -0200
From:   Breno Leitao <leitao@...ian.org>
To:     Mathieu Malaterre <malat@...ian.org>,
        Christophe LEROY <christophe.leroy@....fr>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michael Neuling <michael.neuling@....ibm.com>
Subject: Re: [PATCH] powerpc: Mark variable `cpumsr` as unused

Hi Mathieu, Christophe

Thanks for spotting and fixing this bug.

On 11/08/2018 05:25 AM, Mathieu Malaterre wrote:
> On Thu, Nov 8, 2018 at 7:09 AM Christophe Leroy <christophe.leroy@....fr> wrote:
>>
>>
>>
>> On 11/07/2018 08:26 PM, Mathieu Malaterre wrote:
>>> Add gcc attribute unused for `cpumsr` variable.
>>>
>>> Fix warnings treated as errors with W=1:
>>>
>>>    arch/powerpc/kernel/process.c:231:16: error: variable ‘cpumsr’ set but not used [-Werror=unused-but-set-variable]
>>>    arch/powerpc/kernel/process.c:296:16: error: variable ‘cpumsr’ set but not used [-Werror=unused-but-set-variable]
>>>
>>> Signed-off-by: Mathieu Malaterre <malat@...ian.org>
>>
>> I don't think this is the good way to fix that. This problem was
>> introduced by commit 5c784c8414fb ("powerpc/tm: Remove
>> msr_tm_active()"). That commit should be reverted and fixed.
> 
> I see, it makes sense.
> 
>> That commit should have removed the macro and kept the inline function.
> 
> Breno, what do you think ?

Turning this macro into a function might cause the code to be more confused,
since all the other TM states bits are checked using a macro, for example:

 MSR_TM_SUSPENDED     Checks if the MSR has Suspended bits set
 MSR_TM_TRANSACTIONAL Checks if the MSR has the transactional bits set
 MSR_TM_RESV          Checks if the MSR has the TM reserved bits set	

That said, I understand that it makes sense to have an uniform way to check
for TM bits in MSR, thus having a MSR_TM_ACTIVE macro to check for the active
bits. Using a non-uniform function just to fix this warning seems to be an
overkill. Reverting the patch seems to bring back the old style, which is
having a macro and a function with the same name, where the function just
calls the macro.

Anyway, I think it might have other ways to fix warning, as I can think now:

 1) Avoid setting cpumsr if CONFIG_PPC_TRANSACTIONAL_MEM is not enabled

 2) If !CONFIG_PPC_TRANSACTIONAL_MEM, redefine MSR_TM_ACTIVE(x) to something
as (x & 0) instead of 0.

 3) Avoid double definition of MSR_TM_ACTIVE, i.e, have the same definition
independent of PPC_TRANSACTIONAL_MEM being set or not.

Anyway, I would like to try option 3), which is the hardest one to implement
and validate, but it seems to be the most correct option, once it checks for
a MSR bit configuration, and the caller should have the logic.