[v3] eal: use c11 atomic built-ins for interrupt status
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Message ID 1594283675-15965-1-git-send-email-phil.yang@arm.com
State Accepted, archived
Delegated to: David Marchand
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  • [v3] eal: use c11 atomic built-ins for interrupt status
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Commit Message

Phil Yang July 9, 2020, 8:34 a.m. UTC
The event status is defined as a volatile variable and shared between
threads. Use c11 atomic built-ins with explicit ordering instead of
rte_atomic ops which enforce unnecessary barriers on aarch64.

The event status has been cleaned up by the compare-and-swap operation
when we free the event data, so there is no need to set it to invalid
after that.

Signed-off-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Harman Kalra <hkalra@marvell.com>
---
v3:
Fixed typo.

v2:
1. Fixed typo.
2. Updated libabigail.abignore to pass ABI check.
3. Merged v1 two patches into one patch.

 devtools/libabigail.abignore                |  4 +++
 lib/librte_eal/include/rte_eal_interrupts.h |  2 +-
 lib/librte_eal/linux/eal_interrupts.c       | 48 ++++++++++++++++++++---------
 3 files changed, 38 insertions(+), 16 deletions(-)

Comments

David Marchand July 9, 2020, 10:30 a.m. UTC | #1
On Thu, Jul 9, 2020 at 10:35 AM Phil Yang <phil.yang@arm.com> wrote:
>
> The event status is defined as a volatile variable and shared between
> threads. Use c11 atomic built-ins with explicit ordering instead of
> rte_atomic ops which enforce unnecessary barriers on aarch64.
>
> The event status has been cleaned up by the compare-and-swap operation
> when we free the event data, so there is no need to set it to invalid
> after that.
>
> Signed-off-by: Phil Yang <phil.yang@arm.com>
> Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> Reviewed-by: Harman Kalra <hkalra@marvell.com>
> ---
> v3:
> Fixed typo.
>
> v2:
> 1. Fixed typo.
> 2. Updated libabigail.abignore to pass ABI check.
> 3. Merged v1 two patches into one patch.
>
>  devtools/libabigail.abignore                |  4 +++
>  lib/librte_eal/include/rte_eal_interrupts.h |  2 +-
>  lib/librte_eal/linux/eal_interrupts.c       | 48 ++++++++++++++++++++---------
>  3 files changed, 38 insertions(+), 16 deletions(-)
>
> diff --git a/devtools/libabigail.abignore b/devtools/libabigail.abignore
> index 0133f75..daa4631 100644
> --- a/devtools/libabigail.abignore
> +++ b/devtools/libabigail.abignore
> @@ -48,6 +48,10 @@
>          changed_enumerators = RTE_CRYPTO_AEAD_LIST_END
>  [suppress_variable]
>          name = rte_crypto_aead_algorithm_strings
> +; Ignore updates of epoll event
> +[suppress_type]
> +        type_kind = struct
> +        name = rte_epoll_event

In general, ignoring all changes on a structure is risky.
But the risk is acceptable as long as we remember this for the rest of
the 20.08 release (and we will start from scratch for 20.11).


Without any comment from others, I'll merge this by the end of (my) day.

Thanks.
David Marchand July 10, 2020, 6:32 a.m. UTC | #2
On Thu, Jul 9, 2020 at 10:35 AM Phil Yang <phil.yang@arm.com> wrote:
>
> The event status is defined as a volatile variable and shared between
> threads. Use c11 atomic built-ins with explicit ordering instead of
> rte_atomic ops which enforce unnecessary barriers on aarch64.
>
> The event status has been cleaned up by the compare-and-swap operation
> when we free the event data, so there is no need to set it to invalid
> after that.
>
> Signed-off-by: Phil Yang <phil.yang@arm.com>
> Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> Reviewed-by: Harman Kalra <hkalra@marvell.com>

Applied, thanks.
Dodji Seketeli July 10, 2020, 7:18 a.m. UTC | #3
David Marchand <david.marchand@redhat.com> writes:

[...]

>> --- a/devtools/libabigail.abignore
>> +++ b/devtools/libabigail.abignore
>> @@ -48,6 +48,10 @@
>>          changed_enumerators = RTE_CRYPTO_AEAD_LIST_END
>>  [suppress_variable]
>>          name = rte_crypto_aead_algorithm_strings
>> +; Ignore updates of epoll event
>> +[suppress_type]
>> +        type_kind = struct
>> +        name = rte_epoll_event
>
> In general, ignoring all changes on a structure is risky.
> But the risk is acceptable as long as we remember this for the rest of
> the 20.08 release (and we will start from scratch for 20.11).

Right, I thought about this too when I saw that change.  If that struct
is inherently *not* part of the logically exposed ABI, the risk is
really minimal as well.  In that case, maybe a comment saying so in the
.abignore file could be useful for future reference.

[...]

Cheers,

Patch
diff mbox series

diff --git a/devtools/libabigail.abignore b/devtools/libabigail.abignore
index 0133f75..daa4631 100644
--- a/devtools/libabigail.abignore
+++ b/devtools/libabigail.abignore
@@ -48,6 +48,10 @@ 
         changed_enumerators = RTE_CRYPTO_AEAD_LIST_END
 [suppress_variable]
         name = rte_crypto_aead_algorithm_strings
+; Ignore updates of epoll event
+[suppress_type]
+        type_kind = struct
+        name = rte_epoll_event
 
 ;;;;;;;;;;;;;;;;;;;;;;
 ; Temporary exceptions till DPDK 20.11
diff --git a/lib/librte_eal/include/rte_eal_interrupts.h b/lib/librte_eal/include/rte_eal_interrupts.h
index 773a34a..b1e8a29 100644
--- a/lib/librte_eal/include/rte_eal_interrupts.h
+++ b/lib/librte_eal/include/rte_eal_interrupts.h
@@ -59,7 +59,7 @@  enum {
 
 /** interrupt epoll event obj, taken by epoll_event.ptr */
 struct rte_epoll_event {
-	volatile uint32_t status;  /**< OUT: event status */
+	uint32_t status;           /**< OUT: event status */
 	int fd;                    /**< OUT: event fd */
 	int epfd;       /**< OUT: epoll instance the ev associated with */
 	struct rte_epoll_data epdata;
diff --git a/lib/librte_eal/linux/eal_interrupts.c b/lib/librte_eal/linux/eal_interrupts.c
index 84eeaa1..ad09049 100644
--- a/lib/librte_eal/linux/eal_interrupts.c
+++ b/lib/librte_eal/linux/eal_interrupts.c
@@ -26,7 +26,6 @@ 
 #include <rte_eal.h>
 #include <rte_per_lcore.h>
 #include <rte_lcore.h>
-#include <rte_atomic.h>
 #include <rte_branch_prediction.h>
 #include <rte_debug.h>
 #include <rte_log.h>
@@ -1221,11 +1220,18 @@  eal_epoll_process_event(struct epoll_event *evs, unsigned int n,
 {
 	unsigned int i, count = 0;
 	struct rte_epoll_event *rev;
+	uint32_t valid_status;
 
 	for (i = 0; i < n; i++) {
 		rev = evs[i].data.ptr;
-		if (!rev || !rte_atomic32_cmpset(&rev->status, RTE_EPOLL_VALID,
-						 RTE_EPOLL_EXEC))
+		valid_status =  RTE_EPOLL_VALID;
+		/* ACQUIRE memory ordering here pairs with RELEASE
+		 * ordering below acting as a lock to synchronize
+		 * the event data updating.
+		 */
+		if (!rev || !__atomic_compare_exchange_n(&rev->status,
+				    &valid_status, RTE_EPOLL_EXEC, 0,
+				    __ATOMIC_ACQUIRE, __ATOMIC_RELAXED))
 			continue;
 
 		events[count].status        = RTE_EPOLL_VALID;
@@ -1237,8 +1243,11 @@  eal_epoll_process_event(struct epoll_event *evs, unsigned int n,
 			rev->epdata.cb_fun(rev->fd,
 					   rev->epdata.cb_arg);
 
-		rte_compiler_barrier();
-		rev->status = RTE_EPOLL_VALID;
+		/* the status update should be observed after
+		 * the other fields change.
+		 */
+		__atomic_store_n(&rev->status, RTE_EPOLL_VALID,
+				__ATOMIC_RELEASE);
 		count++;
 	}
 	return count;
@@ -1308,10 +1317,14 @@  rte_epoll_wait(int epfd, struct rte_epoll_event *events,
 static inline void
 eal_epoll_data_safe_free(struct rte_epoll_event *ev)
 {
-	while (!rte_atomic32_cmpset(&ev->status, RTE_EPOLL_VALID,
-				    RTE_EPOLL_INVALID))
-		while (ev->status != RTE_EPOLL_VALID)
+	uint32_t valid_status = RTE_EPOLL_VALID;
+	while (!__atomic_compare_exchange_n(&ev->status, &valid_status,
+		    RTE_EPOLL_INVALID, 0, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED)) {
+		while (__atomic_load_n(&ev->status,
+				__ATOMIC_RELAXED) != RTE_EPOLL_VALID)
 			rte_pause();
+		valid_status = RTE_EPOLL_VALID;
+	}
 	memset(&ev->epdata, 0, sizeof(ev->epdata));
 	ev->fd = -1;
 	ev->epfd = -1;
@@ -1333,7 +1346,8 @@  rte_epoll_ctl(int epfd, int op, int fd,
 		epfd = rte_intr_tls_epfd();
 
 	if (op == EPOLL_CTL_ADD) {
-		event->status = RTE_EPOLL_VALID;
+		__atomic_store_n(&event->status, RTE_EPOLL_VALID,
+				__ATOMIC_RELAXED);
 		event->fd = fd;  /* ignore fd in event */
 		event->epfd = epfd;
 		ev.data.ptr = (void *)event;
@@ -1345,11 +1359,13 @@  rte_epoll_ctl(int epfd, int op, int fd,
 			op, fd, strerror(errno));
 		if (op == EPOLL_CTL_ADD)
 			/* rollback status when CTL_ADD fail */
-			event->status = RTE_EPOLL_INVALID;
+			__atomic_store_n(&event->status, RTE_EPOLL_INVALID,
+					__ATOMIC_RELAXED);
 		return -1;
 	}
 
-	if (op == EPOLL_CTL_DEL && event->status != RTE_EPOLL_INVALID)
+	if (op == EPOLL_CTL_DEL && __atomic_load_n(&event->status,
+			__ATOMIC_RELAXED) != RTE_EPOLL_INVALID)
 		eal_epoll_data_safe_free(event);
 
 	return 0;
@@ -1378,7 +1394,8 @@  rte_intr_rx_ctl(struct rte_intr_handle *intr_handle, int epfd,
 	case RTE_INTR_EVENT_ADD:
 		epfd_op = EPOLL_CTL_ADD;
 		rev = &intr_handle->elist[efd_idx];
-		if (rev->status != RTE_EPOLL_INVALID) {
+		if (__atomic_load_n(&rev->status,
+				__ATOMIC_RELAXED) != RTE_EPOLL_INVALID) {
 			RTE_LOG(INFO, EAL, "Event already been added.\n");
 			return -EEXIST;
 		}
@@ -1401,7 +1418,8 @@  rte_intr_rx_ctl(struct rte_intr_handle *intr_handle, int epfd,
 	case RTE_INTR_EVENT_DEL:
 		epfd_op = EPOLL_CTL_DEL;
 		rev = &intr_handle->elist[efd_idx];
-		if (rev->status == RTE_EPOLL_INVALID) {
+		if (__atomic_load_n(&rev->status,
+				__ATOMIC_RELAXED) == RTE_EPOLL_INVALID) {
 			RTE_LOG(INFO, EAL, "Event does not exist.\n");
 			return -EPERM;
 		}
@@ -1426,12 +1444,12 @@  rte_intr_free_epoll_fd(struct rte_intr_handle *intr_handle)
 
 	for (i = 0; i < intr_handle->nb_efd; i++) {
 		rev = &intr_handle->elist[i];
-		if (rev->status == RTE_EPOLL_INVALID)
+		if (__atomic_load_n(&rev->status,
+				__ATOMIC_RELAXED) == RTE_EPOLL_INVALID)
 			continue;
 		if (rte_epoll_ctl(rev->epfd, EPOLL_CTL_DEL, rev->fd, rev)) {
 			/* force free if the entry valid */
 			eal_epoll_data_safe_free(rev);
-			rev->status = RTE_EPOLL_INVALID;
 		}
 	}
 }