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Date:   Fri, 18 Jun 2021 12:58:07 -0700
From:   "Andy Lutomirski" <luto@kernel.org>
To:     "Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>
Cc:     "the arch/x86 maintainers" <x86@kernel.org>,
        "Dave Hansen" <dave.hansen@intel.com>,
        "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        "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 (Intel)" <peterz@infradead.org>,
        stable <stable@vger.kernel.org>
Subject: =?UTF-8?Q?Re:_[PATCH_8/8]_membarrier:_Rewrite_sync=5Fcore=5Fbefore=5Fuse?=
 =?UTF-8?Q?rmode()_and_improve_documentation?=
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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 wro=
te:
>=20
> > On 6/17/21 7:47 AM, Mathieu Desnoyers wrote:
> >=20
> >> Please change back this #ifndef / #else / #endif within function fo=
r
> >>=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 r=
eadable.
> >=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_ENAB=
LED
> > 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, t=
hat
> > 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_REA=
DY))
>                         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=
_core
> function in the #else case after the ipi_sync_core() function definiti=
on.
>=20
> Or am I missing your point ?

Maybe?

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 co=
mpiler statically verify that it=E2=80=99s not called, but I=E2=80=99m u=
ncomfortable having it there if the implementation is actively incorrect=
.