[PATCH 8/8] membarrier: Rewrite sync_core_before_usermode() and improve documentation

[Nicholas Piggin]

Excerpts from Mathieu Desnoyers's message of June 19, 2021 6:09 am:
> ----- On Jun 18, 2021, at 3:58 PM, Andy Lutomirski luto at 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 at 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