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

Excerpts from Andy Lutomirski's message of June 20, 2021 1:50 am:

On Fri, Jun 18, 2021, at 11:02 PM, Nicholas Piggin wrote:

quoted

Excerpts from Mathieu Desnoyers's message of June 19, 2021 6:09 am:

quoted

----- On Jun 18, 2021, at 3:58 PM, Andy Lutomirski luto@kernel.org wrote:

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On Fri, Jun 18, 2021, at 9:31 AM, Mathieu Desnoyers wrote:

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----- On Jun 17, 2021, at 8:12 PM, Andy Lutomirski luto@kernel.org wrote:

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On 6/17/21 7:47 AM, Mathieu Desnoyers wrote:

quoted

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.

It had better, because compiletime_assert already relies on a similar 
level of code elimination to work.

I think it's fine to use for now, but it may not be quite the the 
logically correct primitive if we want to be really clean, because a 
valid compiletime_assert implementation should be able to fire even for 
code that is never linked. We would want something like to be clean 
IMO:

#ifdef CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE
#include <asm/sync_core.h>
#else
extern void membarrier_sync_core_before_usermode(void) __compiletime_error("architecture does not implement membarrier_sync_core_before_usermode");
#endif

However that does not have the ifdef for optimising compile so AFAIKS it 
could break with a false positive in some cases.

Something like compiletime_assert_not_called("msg") that either compiles
to a noop or a __compiletime_error depending on __OPTIMIZE__ would be 
the way to go IMO. I don't know if anything exists that fits, but it's
certainly not a unique thing in the kernel so I may not be looking hard
enough.

Thanks,
Nick