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Subject: Re: [PATCH 8/8] membarrier: Rewrite sync_core_before_usermode() and
 improve documentation
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     x86 <x86@kernel.org>, Dave Hansen <dave.hansen@intel.com>,
        linux-kernel <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 <peterz@infradead.org>,
        stable <stable@vger.kernel.org>
References: <cover.1623813516.git.luto@kernel.org>
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From:   Andy Lutomirski <luto@kernel.org>
Message-ID: <26196903-4aee-33c4-bed8-8bf8c7b46793@kernel.org>
Date:   Thu, 17 Jun 2021 17:12:15 -0700
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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();
}