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Date:   Fri, 18 Jun 2021 16:09:41 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Andy Lutomirski <luto@kernel.org>
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        linux-kernel <linux-kernel@vger.kernel.org>,
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        Andrew Morton <akpm@linux-foundation.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        Peter Zijlstra <peterz@infradead.org>,
        stable <stable@vger.kernel.org>
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Subject: Re: [PATCH 8/8] membarrier: Rewrite sync_core_before_usermode() and
 improve documentation
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Thread-Topic: membarrier: Rewrite sync_core_before_usermode() and improve documentation
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----- On Jun 18, 2021, at 3:58 PM, Andy Lutomirski luto@kernel.org wrote:

> On Fri, Jun 18, 2021, at 9:31 AM, Mathieu Desnoyers wrote:
>> ----- On Jun 17, 2021, at 8:12 PM, Andy Lutomirski luto@kernel.org wrote=
:
>>=20
>> > On 6/17/21 7:47 AM, Mathieu Desnoyers wrote:
>> >=20
>> >> Please change back this #ifndef / #else / #endif within function for
>> >>=20
>> >> if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE)) {
>> >>   ...
>> >> } else {
>> >>   ...
>> >> }
>> >>=20
>> >> I don't think mixing up preprocessor and code logic makes it more rea=
dable.
>> >=20
>> > I agree, but I don't know how to make the result work well.
>> > membarrier_sync_core_before_usermode() isn't defined in the !IS_ENABLE=
D
>> > case, so either I need to fake up a definition or use #ifdef.
>> >=20
>> > If I faked up a definition, I would want to assert, at build time, tha=
t
>> > it isn't called.  I don't think we can do:
>> >=20
>> > static void membarrier_sync_core_before_usermode()
>> > {
>> >    BUILD_BUG_IF_REACHABLE();
>> > }
>>=20
>> Let's look at the context here:
>>=20
>> static void ipi_sync_core(void *info)
>> {
>>     [....]
>>     membarrier_sync_core_before_usermode()
>> }
>>=20
>> ^ this can be within #ifdef / #endif
>>=20
>> static int membarrier_private_expedited(int flags, int cpu_id)
>> [...]
>>                if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE))
>>                         return -EINVAL;
>>                 if (!(atomic_read(&mm->membarrier_state) &
>>                       MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY=
))
>>                         return -EPERM;
>>                 ipi_func =3D ipi_sync_core;
>>=20
>> All we need to make the line above work is to define an empty ipi_sync_c=
ore
>> function in the #else case after the ipi_sync_core() function definition=
.
>>=20
>> Or am I missing your point ?
>=20
> Maybe?
>=20
> My objection is that an empty ipi_sync_core is a lie =E2=80=94 it doesn=
=E2=80=99t sync the core.
> I would be fine with that if I could have the compiler statically verify =
that
> it=E2=80=99s not called, but I=E2=80=99m uncomfortable having it there if=
 the implementation is
> actively incorrect.

I see. Another approach would be to implement a "setter" function to popula=
te
"ipi_func". That setter function would return -EINVAL in its #ifndef CONFIG=
_ARCH_HAS_MEMBARRIER_SYNC_CORE
implementation.

Would that be better ?

Thanks,

Mathieu

--=20
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com