From: Nicholas Piggin <npiggin@gmail.com>
To: Andy Lutomirski <luto@kernel.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Dave Hansen <dave.hansen@intel.com>,
linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
linux-mm <linux-mm@kvack.org>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
Michael Ellerman <mpe@ellerman.id.au>,
Paul Mackerras <paulus@samba.org>,
Peter Zijlstra <peterz@infradead.org>,
stable <stable@vger.kernel.org>, Will Deacon <will@kernel.org>,
x86 <x86@kernel.org>
Subject: Re: [PATCH 8/8] membarrier: Rewrite sync_core_before_usermode() and improve documentation
Date: Sat, 19 Jun 2021 16:02:21 +1000 [thread overview]
Message-ID: <1624080924.z61zvzi4cq.astroid@bobo.none> (raw)
In-Reply-To: <639092151.13266.1624046981084.JavaMail.zimbra@efficios.com>
Excerpts from Mathieu Desnoyers's message of June 19, 2021 6:09 am:
> ----- On Jun 18, 2021, at 3:58 PM, Andy Lutomirski luto@kernel.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@kernel.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");
+}
+
+#endif
/*
* For documentation purposes, here are some membarrier ordering
next prev parent reply other threads:[~2021-06-19 6:02 UTC|newest]
Thread overview: 91+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-16 3:21 [PATCH 0/8] membarrier cleanups Andy Lutomirski
2021-06-16 3:21 ` [PATCH 1/8] membarrier: Document why membarrier() works Andy Lutomirski
2021-06-16 4:00 ` Nicholas Piggin
2021-06-16 7:30 ` Peter Zijlstra
2021-06-17 23:45 ` Andy Lutomirski
2021-06-16 3:21 ` [PATCH 2/8] x86/mm: Handle unlazying membarrier core sync in the arch code Andy Lutomirski
2021-06-16 4:25 ` Nicholas Piggin
2021-06-16 18:31 ` Andy Lutomirski
2021-06-16 17:49 ` Mathieu Desnoyers
2021-06-16 18:31 ` Andy Lutomirski
2021-06-16 3:21 ` [PATCH 3/8] membarrier: Remove membarrier_arch_switch_mm() prototype in core code Andy Lutomirski
2021-06-16 4:26 ` Nicholas Piggin
2021-06-16 17:52 ` Mathieu Desnoyers
2021-06-16 3:21 ` [PATCH 4/8] membarrier: Make the post-switch-mm barrier explicit Andy Lutomirski
2021-06-16 4:19 ` Nicholas Piggin
2021-06-16 7:35 ` Peter Zijlstra
2021-06-16 18:41 ` Andy Lutomirski
2021-06-17 1:37 ` Nicholas Piggin
2021-06-17 2:57 ` Andy Lutomirski
2021-06-17 5:32 ` Andy Lutomirski
2021-06-17 6:51 ` Nicholas Piggin
2021-06-17 23:49 ` Andy Lutomirski
2021-06-19 2:53 ` Nicholas Piggin
2021-06-19 3:20 ` Andy Lutomirski
2021-06-19 4:27 ` Nicholas Piggin
2021-06-17 9:08 ` [RFC][PATCH] sched: Use lightweight hazard pointers to grab lazy mms Peter Zijlstra
2021-06-17 9:10 ` Peter Zijlstra
2021-06-17 10:00 ` Nicholas Piggin
2021-06-17 9:13 ` Peter Zijlstra
2021-06-17 14:06 ` Andy Lutomirski
2021-06-17 9:28 ` Peter Zijlstra
2021-06-17 14:03 ` Andy Lutomirski
2021-06-17 14:10 ` Andy Lutomirski
2021-06-17 15:45 ` Peter Zijlstra
2021-06-18 3:29 ` Paul E. McKenney
2021-06-18 5:04 ` Andy Lutomirski
2021-06-17 15:02 ` [PATCH 4/8] membarrier: Make the post-switch-mm barrier explicit Paul E. McKenney
2021-06-18 0:06 ` Andy Lutomirski
2021-06-18 3:35 ` Paul E. McKenney
2021-06-17 8:45 ` Peter Zijlstra
2021-06-16 3:21 ` [PATCH 5/8] membarrier, kthread: Use _ONCE accessors for task->mm Andy Lutomirski
2021-06-16 4:28 ` Nicholas Piggin
2021-06-16 18:08 ` Mathieu Desnoyers
2021-06-16 18:45 ` Andy Lutomirski
2021-06-16 3:21 ` [PATCH 6/8] powerpc/membarrier: Remove special barrier on mm switch Andy Lutomirski
2021-06-16 4:36 ` Nicholas Piggin
2021-06-16 3:21 ` [PATCH 7/8] membarrier: Remove arm (32) support for SYNC_CORE Andy Lutomirski
2021-06-16 9:28 ` Russell King (Oracle)
2021-06-16 10:16 ` Peter Zijlstra
2021-06-16 10:20 ` Peter Zijlstra
2021-06-16 10:34 ` Russell King (Oracle)
2021-06-16 11:10 ` Peter Zijlstra
2021-06-16 13:22 ` Russell King (Oracle)
2021-06-16 15:04 ` Catalin Marinas
2021-06-16 15:23 ` Russell King (Oracle)
2021-06-16 15:45 ` Catalin Marinas
2021-06-16 16:00 ` Catalin Marinas
2021-06-16 16:27 ` Russell King (Oracle)
2021-06-17 8:55 ` Krzysztof Hałasa
2021-06-18 12:54 ` Linus Walleij
2021-06-18 13:19 ` Russell King (Oracle)
2021-06-18 13:36 ` Arnd Bergmann
2021-06-17 10:40 ` Mark Rutland
2021-06-17 11:23 ` Russell King (Oracle)
2021-06-17 11:33 ` Mark Rutland
2021-06-17 13:41 ` Andy Lutomirski
2021-06-17 13:51 ` Mark Rutland
2021-06-17 14:00 ` Andy Lutomirski
2021-06-17 14:20 ` Mark Rutland
2021-06-17 15:01 ` Peter Zijlstra
2021-06-17 15:13 ` Peter Zijlstra
2021-06-17 14:16 ` Mathieu Desnoyers
2021-06-17 14:05 ` Peter Zijlstra
2021-06-18 0:07 ` Andy Lutomirski
2021-06-16 3:21 ` [PATCH 8/8] membarrier: Rewrite sync_core_before_usermode() and improve documentation Andy Lutomirski
2021-06-16 4:45 ` Nicholas Piggin
2021-06-16 18:52 ` Andy Lutomirski
2021-06-16 23:48 ` Andy Lutomirski
2021-06-18 15:27 ` Christophe Leroy
2021-06-16 10:20 ` Will Deacon
2021-06-16 23:58 ` Andy Lutomirski
2021-06-17 14:47 ` Mathieu Desnoyers
2021-06-18 0:12 ` Andy Lutomirski
2021-06-18 16:31 ` Mathieu Desnoyers
2021-06-18 19:58 ` Andy Lutomirski
2021-06-18 20:09 ` Mathieu Desnoyers
2021-06-19 6:02 ` Nicholas Piggin [this message]
2021-06-19 15:50 ` Andy Lutomirski
2021-06-20 2:10 ` Nicholas Piggin
2021-06-17 15:16 ` Mathieu Desnoyers
2021-06-18 0:13 ` Andy Lutomirski
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