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:
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"); +}
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