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

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Date:   Fri, 18 Jun 2021 12:31:48 -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 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 ?

Thanks,

Mathieu

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