> -----Original Message-----
> From: Phil Yang <phil.y...@arm.com>
> Sent: Thursday, April 23, 2020 5:31 PM
> To: Van Haaren, Harry <harry.van.haa...@intel.com>; dev@dpdk.org
> Cc: tho...@monjalon.net; david.march...@redhat.com; Ananyev, Konstantin
> <konstantin.anan...@intel.com>; jer...@marvell.com;
> hemant.agra...@nxp.com; honnappa.nagaraha...@arm.com;
> gavin...@arm.com; n...@arm.com; Honnappa Nagarahalli
> <honnappa.nagaraha...@arm.com>; sta...@dpdk.org
> Subject: [PATCH v2 1/6] service: fix race condition for MT unsafe service
>
> From: Honnappa Nagarahalli <honnappa.nagaraha...@arm.com>
>
> The MT unsafe service might get configured to run on another core
> while the service is running currently. This might result in the
> MT unsafe service running on multiple cores simultaneously. Use
> 'execute_lock' always when the service is MT unsafe.
>
> Fixes: e9139a32f6e8 ("service: add function to run on app lcore")
> Cc: sta...@dpdk.org
>
> Signed-off-by: Honnappa Nagarahalli <honnappa.nagaraha...@arm.com>
> Reviewed-by: Phil Yang <phil.y...@arm.com>
> ---
Thanks for spinning a new revision - based on ML discussion previously,
it seems like the "use service-run-count" to avoid this race would be a
complex solution.
Suggesting the following;
1) Take the approach as per this patch, to always take the atomic, fixing the
race condition.
2) Add an API to service-cores, which allows "committing" of mappings.
Committing the mapping would imply that the mappings will not be changed in
future. With runtime-remapping being removed from the equation, the existing
branch-over-atomic optimization is valid again.
So this would offer applications two situations
A) No application change: possible performance regression due to atomic always
taken.
B) Call "commit" API, and regain the performance as per previous DPDK versions.
Thoughts/opinions on the above? I've flagged the rest of the patchset for
review ASAP. Regards, -Harry
> lib/librte_eal/common/rte_service.c | 11 +++++------
> 1 file changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/lib/librte_eal/common/rte_service.c
> b/lib/librte_eal/common/rte_service.c
> index 70d17a5..b8c465e 100644
> --- a/lib/librte_eal/common/rte_service.c
> +++ b/lib/librte_eal/common/rte_service.c
> @@ -50,6 +50,10 @@ struct rte_service_spec_impl {
> uint8_t internal_flags;
>
> /* per service statistics */
> + /* Indicates how many cores the service is mapped to run on.
> + * It does not indicate the number of cores the service is running
> + * on currently.
> + */
> rte_atomic32_t num_mapped_cores;
> uint64_t calls;
> uint64_t cycles_spent;
> @@ -370,12 +374,7 @@ service_run(uint32_t i, struct core_state *cs, uint64_t
> service_mask,
>
> cs->service_active_on_lcore[i] = 1;
>
> - /* check do we need cmpset, if MT safe or <= 1 core
> - * mapped, atomic ops are not required.
> - */
> - const int use_atomics = (service_mt_safe(s) == 0) &&
> - (rte_atomic32_read(&s->num_mapped_cores) > 1);
> - if (use_atomics) {
> + if (service_mt_safe(s) == 0) {
> if (!rte_atomic32_cmpset((uint32_t *)&s->execute_lock, 0, 1))
> return -EBUSY;
>
> --
> 2.7.4
