linux-kernel - Re: [PATCH 8/8] membarrier: Rewrite sync_core_before

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Message-Id: <1d617df2-57fa-4953-9c75-6de3909a6f14@www.fastmail.com>
Date:   Fri, 18 Jun 2021 12:58:07 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>
Cc:     "the arch/x86 maintainers" <x86@...nel.org>,
        "Dave Hansen" <dave.hansen@...el.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Michael Ellerman" <mpe@...erman.id.au>,
        "Benjamin Herrenschmidt" <benh@...nel.crashing.org>,
        "Paul Mackerras" <paulus@...ba.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        "Nicholas Piggin" <npiggin@...il.com>,
        "Catalin Marinas" <catalin.marinas@....com>,
        "Will Deacon" <will@...nel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH 8/8] membarrier: Rewrite sync_core_before_usermode() and improve documentation



On Fri, Jun 18, 2021, at 9:31 AM, Mathieu Desnoyers wrote:
> ----- On Jun 17, 2021, at 8:12 PM, Andy Lutomirski luto@...nel.org wrote:
> 
> > On 6/17/21 7:47 AM, Mathieu Desnoyers wrote:
> > 
> >> Please change back this #ifndef / #else / #endif within function for
> >> 
> >> if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE)) {
> >>   ...
> >> } else {
> >>   ...
> >> }
> >> 
> >> I don't think mixing up preprocessor and code logic makes it more readable.
> > 
> > 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_ENABLED
> > case, so either I need to fake up a definition or use #ifdef.
> > 
> > If I faked up a definition, I would want to assert, at build time, that
> > it isn't called.  I don't think we can do:
> > 
> > static void membarrier_sync_core_before_usermode()
> > {
> >    BUILD_BUG_IF_REACHABLE();
> > }
> 
> Let's look at the context here:
> 
> static void ipi_sync_core(void *info)
> {
>     [....]
>     membarrier_sync_core_before_usermode()
> }
> 
> ^ this can be within #ifdef / #endif
> 
> 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 = ipi_sync_core;
> 
> 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 definition.
> 
> Or am I missing your point ?

Maybe?

My objection is that an empty ipi_sync_core is a lie — it doesn’t sync the core.  I would be fine with that if I could have the compiler statically verify that it’s not called, but I’m uncomfortable having it there if the implementation is actively incorrect.