Re: [dpdk-dev] [PATCH v3] lib/timer: relax barrier for status update

[Thomas Monjalon <thomas@monjalon.net>]

24/04/2020 09:24, Phil Yang:
> Volatile has no ordering semantics. The rte_timer structure defines
> timer status as a volatile variable and uses the rte_r/wmb barrier
> to guarantee inter-thread visibility.
> 
> This patch optimized the volatile operation with c11 atomic operations
> and one-way barrier to save the performance penalty. According to the
> timer_perf_autotest benchmarking results, this patch can uplift 10%~16%
> timer appending performance, 3%~20% timer resetting performance and 45%
> timer callbacks scheduling performance on aarch64 and no loss in
> performance for x86.
> 
> Suggested-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> Signed-off-by: Phil Yang <phil.yang@arm.com>
> Reviewed-by: Gavin Hu <gavin.hu@arm.com>
> Acked-by: Erik Gabriel Carrillo <erik.g.carrillo@intel.com>
[...]
> --- a/lib/librte_timer/rte_timer.h
> +++ b/lib/librte_timer/rte_timer.h
> @@ -101,7 +101,7 @@ struct rte_timer
> -	volatile union rte_timer_status status; /**< Status of timer. */
> +	union rte_timer_status status; /**< Status of timer. */

Unfortunately, I cannot merge this patch because it breaks the ABI:

  [C]'function void rte_timer_init(rte_timer*)' at rte_timer.c:214:1 has some indirect sub-type changes:
    parameter 1 of type 'rte_timer*' has sub-type changes:
      in pointed to type 'struct rte_timer' at rte_timer.h:100:1:
        type size hasn't changed
        1 data member changes (2 filtered):
         type of 'volatile rte_timer_status rte_timer::status' changed:
           entity changed from 'volatile rte_timer_status' to 'union rte_timer_status' at rte_timer.h:67:1
           type size hasn't changed