Re: [PATCH v2] eal/arm: replace RTE_BUILD_BUG on non-constant

[Daniel Gregory <daniel.gregory@bytedance.com>]

On Thu, Jun 27, 2024 at 05:08:51PM +0200, Thomas Monjalon wrote:
> 04/05/2024 02:59, Stephen Hemminger:
> > On Fri,  3 May 2024 19:27:30 +0100
> > Daniel Gregory <daniel.gregory@bytedance.com> wrote:
> > 
> > > The ARM implementation of rte_pause uses RTE_BUILD_BUG_ON to check
> > > memorder, which is not constant. This causes compile errors when it is
> > > enabled with RTE_ARM_USE_WFE. eg.
> > > 
> > > ../lib/eal/arm/include/rte_pause_64.h: In function ‘rte_wait_until_equal_16’:
> > > ../lib/eal/include/rte_common.h:530:56: error: expression in static assertion is not constant
> > >   530 | #define RTE_BUILD_BUG_ON(condition) do { static_assert(!(condition), #condition); } while (0)
> > >       |                                                        ^~~~~~~~~~~~
> > > ../lib/eal/arm/include/rte_pause_64.h:156:9: note: in expansion of macro ‘RTE_BUILD_BUG_ON’
> > >   156 |         RTE_BUILD_BUG_ON(memorder != rte_memory_order_acquire &&
> > >       |         ^~~~~~~~~~~~~~~~
> > > 
> > > Fix the compile errors by replacing the check with an assert, like in
> > > the generic implementation (lib/eal/include/generic/rte_pause.h).
> > 
> > No, don't hide the problem.
> > 
> > What code is calling these. Looks like a real bug. Could be behind layers of wrappers.
> 
> I support Stephen's opinion.
> Please look for the real issue.

In DPDK, I have found 26 calls of rte_wait_until_equal_16, largely split
between app/test-bbdev/test_bbdev_perf.c and app/test/test_timer.c, with
a couple calls in lib/eal/include/rte_pflock.h and
lib/eal/include/rte_ticketlock.h as well. 16 calls of
rte_wait_until_equal_32, spread amongst various test cases
(test_func_reentrancy.c test_mcslock.c, test_mempool_perf.c, ...), two
drivers (drivers/event/opdl/opdl_ring.c and
drivers/net/thunderx/nicvf_rxrx.c), lib/eal/common/eal_common_mcfg.c,
lib/eal/include/generic/rte_spinlock.h, lib/ring/rte_ring_c11_pvt.h,
lib/ring/rte_ring_generic_pvt.h and lib/eal/include/rte_mcslock.h. There
is a single call to rte_wait_until_equal_64 in app/test/test_pmd_perf.c

They all correctly use the primitives from rte_stdatomic.h

As I discussed on another chain
https://lore.kernel.org/dpdk-dev/20240509110251.GA3795959@ste-uk-lab-gw/
from what I've seen, it seems that neither Clang nor GCC allow for
static checks on the parameters of inline functions. For instance, the
following does not compile:

static inline __attribute__((always_inline)) int
fn(int val)
{
	_Static_assert(val == 0, "val nonzero");
	return 0;
}

int main(void) {
	return fn(0);
}

( https://godbolt.org/z/TrfWqYoGo )

With the same "expression in static assertion is not constant" error
that I get when cross-compiling DPDK for ARM with WFE enabled on main:

diff --git a/config/arm/meson.build b/config/arm/meson.build
index a45aa9e466..661c735977 100644
--- a/config/arm/meson.build
+++ b/config/arm/meson.build
@@ -18,7 +18,7 @@ flags_common = [
         #    ['RTE_ARM64_MEMCPY_STRICT_ALIGN', false],
 
         # Enable use of ARM wait for event instruction.
-        # ['RTE_ARM_USE_WFE', false],
+        ['RTE_ARM_USE_WFE', true],
 
         ['RTE_ARCH_ARM64', true],
         ['RTE_CACHE_LINE_SIZE', 128]