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

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Message-ID: <639092151.13266.1624046981084.JavaMail.zimbra@efficios.com>
Date:   Fri, 18 Jun 2021 16:09:41 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     x86 <x86@...nel.org>, Dave Hansen <dave.hansen@...el.com>,
        linux-kernel <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 <peterz@...radead.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH 8/8] membarrier: Rewrite sync_core_before_usermode() and
 improve documentation

----- On Jun 18, 2021, at 3:58 PM, Andy Lutomirski luto@...nel.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@...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.

I see. Another approach would be to implement a "setter" function to populate
"ipi_func". That setter function would return -EINVAL in its #ifndef CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE
implementation.

Would that be better ?

Thanks,

Mathieu

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