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

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Message-Id: <2f9e52eb-0105-4bc6-a903-f4ecbfc9b999@www.fastmail.com>
Date:   Sat, 19 Jun 2021 08:50:41 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Nicholas Piggin" <npiggin@...il.com>,
        "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>
Cc:     "Andrew Morton" <akpm@...ux-foundation.org>,
        "Benjamin Herrenschmidt" <benh@...nel.crashing.org>,
        "Catalin Marinas" <catalin.marinas@....com>,
        "Dave Hansen" <dave.hansen@...el.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        "Michael Ellerman" <mpe@...erman.id.au>,
        "Paul Mackerras" <paulus@...ba.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        stable <stable@...r.kernel.org>, "Will Deacon" <will@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH 8/8] membarrier: Rewrite sync_core_before_usermode() and improve documentation



On Fri, Jun 18, 2021, at 11:02 PM, Nicholas Piggin wrote:
> Excerpts from Mathieu Desnoyers's message of June 19, 2021 6:09 am:
> > ----- 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.
> 
> I still don't get the problem with my suggestion. Sure the 
> ipi is a "lie", but it doesn't get used. That's how a lot of
> ifdef folding works out. E.g.,
> 
> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
> index b5add64d9698..54cb32d064af 100644
> --- a/kernel/sched/membarrier.c
> +++ b/kernel/sched/membarrier.c
> @@ -5,6 +5,15 @@
>   * membarrier system call
>   */
>  #include "sched.h"
> +#ifdef CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE
> +#include <asm/sync_core.h>
> +#else
> +static inline void membarrier_sync_core_before_usermode(void)
> +{
> +	compiletime_assert(0, "architecture does not implement 
> membarrier_sync_core_before_usermode");
> +}
> +

With the assert there, I’m fine with this. Let me see if the result builds.

> +#endif
>  
>  /*
>   * For documentation purposes, here are some membarrier ordering
>