Re: [dpdk-dev] [PATCH v3 4/4] eventdev: relax smp barriers with C11 atomics

[Jerin Jacob <jerinjacobk@gmail.com>]

On Tue, Jul 7, 2020 at 4:45 PM Phil Yang <phil.yang@arm.com> wrote:
>
> The impl_opaque field is shared between the timer arm and cancel
> operations. Meanwhile, the state flag acts as a guard variable to
> make sure the update of impl_opaque is synchronized. The original
> code uses rte_smp barriers to achieve that. This patch uses C11
> atomics with an explicit one-way memory barrier instead of full
> barriers rte_smp_w/rmb() to avoid the unnecessary barrier on aarch64.
>
> Since compilers can generate the same instructions for volatile and
> non-volatile variable in C11 __atomics built-ins, so remain the volatile
> keyword in front of state enum to avoid the ABI break issue.
>
> Signed-off-by: Phil Yang <phil.yang@arm.com>
> Reviewed-by: Dharmik Thakkar <dharmik.thakkar@arm.com>
> Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> Acked-by: Erik Gabriel Carrillo <erik.g.carrillo@intel.com>


Could you fix the following:

WARNING:TYPO_SPELLING: 'opague' may be misspelled - perhaps 'opaque'?
#184: FILE: lib/librte_eventdev/rte_event_timer_adapter.c:1161:
+ * specific opague data under the correct state.

total: 0 errors, 1 warnings, 124 lines checked


> ---
> v3:
> Fix ABI issue: revert to 'volatile enum rte_event_timer_state type state'.
>
> v2:
> 1. Removed implementation-specific opaque data cleanup code.
> 2. Replaced thread fence with atomic ACQURE/RELEASE ordering on state access.
>
>  lib/librte_eventdev/rte_event_timer_adapter.c | 55 ++++++++++++++++++---------
>  1 file changed, 37 insertions(+), 18 deletions(-)
>
> diff --git a/lib/librte_eventdev/rte_event_timer_adapter.c b/lib/librte_eventdev/rte_event_timer_adapter.c
> index d75415c..eb2c93a 100644
> --- a/lib/librte_eventdev/rte_event_timer_adapter.c
> +++ b/lib/librte_eventdev/rte_event_timer_adapter.c
> @@ -629,7 +629,8 @@ swtim_callback(struct rte_timer *tim)
>                 sw->expired_timers[sw->n_expired_timers++] = tim;
>                 sw->stats.evtim_exp_count++;
>
> -               evtim->state = RTE_EVENT_TIMER_NOT_ARMED;
> +               __atomic_store_n(&evtim->state, RTE_EVENT_TIMER_NOT_ARMED,
> +                               __ATOMIC_RELEASE);
>         }
>
>         if (event_buffer_batch_ready(&sw->buffer)) {
> @@ -1020,6 +1021,7 @@ __swtim_arm_burst(const struct rte_event_timer_adapter *adapter,
>         int n_lcores;
>         /* Timer list for this lcore is not in use. */
>         uint16_t exp_state = 0;
> +       enum rte_event_timer_state n_state;
>
>  #ifdef RTE_LIBRTE_EVENTDEV_DEBUG
>         /* Check that the service is running. */
> @@ -1060,30 +1062,36 @@ __swtim_arm_burst(const struct rte_event_timer_adapter *adapter,
>         }
>
>         for (i = 0; i < nb_evtims; i++) {
> -               /* Don't modify the event timer state in these cases */
> -               if (evtims[i]->state == RTE_EVENT_TIMER_ARMED) {
> +               n_state = __atomic_load_n(&evtims[i]->state, __ATOMIC_ACQUIRE);
> +               if (n_state == RTE_EVENT_TIMER_ARMED) {
>                         rte_errno = EALREADY;
>                         break;
> -               } else if (!(evtims[i]->state == RTE_EVENT_TIMER_NOT_ARMED ||
> -                            evtims[i]->state == RTE_EVENT_TIMER_CANCELED)) {
> +               } else if (!(n_state == RTE_EVENT_TIMER_NOT_ARMED ||
> +                            n_state == RTE_EVENT_TIMER_CANCELED)) {
>                         rte_errno = EINVAL;
>                         break;
>                 }
>
>                 ret = check_timeout(evtims[i], adapter);
>                 if (unlikely(ret == -1)) {
> -                       evtims[i]->state = RTE_EVENT_TIMER_ERROR_TOOLATE;
> +                       __atomic_store_n(&evtims[i]->state,
> +                                       RTE_EVENT_TIMER_ERROR_TOOLATE,
> +                                       __ATOMIC_RELAXED);
>                         rte_errno = EINVAL;
>                         break;
>                 } else if (unlikely(ret == -2)) {
> -                       evtims[i]->state = RTE_EVENT_TIMER_ERROR_TOOEARLY;
> +                       __atomic_store_n(&evtims[i]->state,
> +                                       RTE_EVENT_TIMER_ERROR_TOOEARLY,
> +                                       __ATOMIC_RELAXED);
>                         rte_errno = EINVAL;
>                         break;
>                 }
>
>                 if (unlikely(check_destination_event_queue(evtims[i],
>                                                            adapter) < 0)) {
> -                       evtims[i]->state = RTE_EVENT_TIMER_ERROR;
> +                       __atomic_store_n(&evtims[i]->state,
> +                                       RTE_EVENT_TIMER_ERROR,
> +                                       __ATOMIC_RELAXED);
>                         rte_errno = EINVAL;
>                         break;
>                 }
> @@ -1099,13 +1107,18 @@ __swtim_arm_burst(const struct rte_event_timer_adapter *adapter,
>                                           SINGLE, lcore_id, NULL, evtims[i]);
>                 if (ret < 0) {
>                         /* tim was in RUNNING or CONFIG state */
> -                       evtims[i]->state = RTE_EVENT_TIMER_ERROR;
> +                       __atomic_store_n(&evtims[i]->state,
> +                                       RTE_EVENT_TIMER_ERROR,
> +                                       __ATOMIC_RELEASE);
>                         break;
>                 }
>
> -               rte_smp_wmb();
>                 EVTIM_LOG_DBG("armed an event timer");
> -               evtims[i]->state = RTE_EVENT_TIMER_ARMED;
> +               /* RELEASE ordering guarantees the adapter specific value
> +                * changes observed before the update of state.
> +                */
> +               __atomic_store_n(&evtims[i]->state, RTE_EVENT_TIMER_ARMED,
> +                               __ATOMIC_RELEASE);
>         }
>
>         if (i < nb_evtims)
> @@ -1132,6 +1145,7 @@ swtim_cancel_burst(const struct rte_event_timer_adapter *adapter,
>         struct rte_timer *timp;
>         uint64_t opaque;
>         struct swtim *sw = swtim_pmd_priv(adapter);
> +       enum rte_event_timer_state n_state;
>
>  #ifdef RTE_LIBRTE_EVENTDEV_DEBUG
>         /* Check that the service is running. */
> @@ -1143,16 +1157,18 @@ swtim_cancel_burst(const struct rte_event_timer_adapter *adapter,
>
>         for (i = 0; i < nb_evtims; i++) {
>                 /* Don't modify the event timer state in these cases */
> -               if (evtims[i]->state == RTE_EVENT_TIMER_CANCELED) {
> +               /* ACQUIRE ordering guarantees the access of implementation
> +                * specific opague data under the correct state.
> +                */
> +               n_state = __atomic_load_n(&evtims[i]->state, __ATOMIC_ACQUIRE);
> +               if (n_state == RTE_EVENT_TIMER_CANCELED) {
>                         rte_errno = EALREADY;
>                         break;
> -               } else if (evtims[i]->state != RTE_EVENT_TIMER_ARMED) {
> +               } else if (n_state != RTE_EVENT_TIMER_ARMED) {
>                         rte_errno = EINVAL;
>                         break;
>                 }
>
> -               rte_smp_rmb();
> -
>                 opaque = evtims[i]->impl_opaque[0];
>                 timp = (struct rte_timer *)(uintptr_t)opaque;
>                 RTE_ASSERT(timp != NULL);
> @@ -1166,9 +1182,12 @@ swtim_cancel_burst(const struct rte_event_timer_adapter *adapter,
>
>                 rte_mempool_put(sw->tim_pool, (void **)timp);
>
> -               evtims[i]->state = RTE_EVENT_TIMER_CANCELED;
> -
> -               rte_smp_wmb();
> +               /* The RELEASE ordering here pairs with atomic ordering
> +                * to make sure the state update data observed between
> +                * threads.
> +                */
> +               __atomic_store_n(&evtims[i]->state, RTE_EVENT_TIMER_CANCELED,
> +                               __ATOMIC_RELEASE);
>         }
>
>         return i;
> --
> 2.7.4
>