[PATCH 2/9] eal/interrupts: keep real errno on epoll error

[Maxime Leroy]

Some interrupt users have several vectors backed by the same eventfd
(e.g. several Rx queues behind one DPAA2 portal eventfd). Adding the
second vector to the same epoll instance then fails with EEXIST.

Upper layers such as ethdev and bbdev already treat -EEXIST as a
non-fatal duplicate registration (if (ret && ret != -EEXIST)), but
rte_intr_rx_ctl() lost that information: rte_epoll_ctl() returned -1 and
rte_intr_rx_ctl() flattened every failure to -EPERM.

Return the negative errno from rte_epoll_ctl() (its documented contract
is already "a negative value") and stop rte_intr_rx_ctl() from
flattening errors to -EPERM, so EEXIST reaches the upper layers that
already handle it; other failures carry their real errno.

Fixes: 9efe9c6cdcac ("eal/linux: add epoll wrappers")
Fixes: c9f3ec1a0f3f ("eal/linux: add Rx interrupt control function")
Cc: stable at dpdk.org
Signed-off-by: Maxime Leroy <maxime at leroys.fr>
---
 lib/eal/include/rte_epoll.h    |  3 ++-
 lib/eal/linux/eal_interrupts.c | 18 +++++++++++-------
 2 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/lib/eal/include/rte_epoll.h b/lib/eal/include/rte_epoll.h
index ae0cf20853..0c7b510563 100644
--- a/lib/eal/include/rte_epoll.h
+++ b/lib/eal/include/rte_epoll.h
@@ -104,7 +104,8 @@ rte_epoll_wait_interruptible(int epfd, struct rte_epoll_event *events,
  *   Note: The caller must take care the object deletion after CTL_DEL.
  * @return
  *   - On success, zero.
- *   - On failure, a negative value.
+ *   - On failure, a negative errno value, e.g. -EEXIST if the fd is already
+ *     registered on the epoll instance (a fd shared between vectors).
  */
 int
 rte_epoll_ctl(int epfd, int op, int fd,
diff --git a/lib/eal/linux/eal_interrupts.c b/lib/eal/linux/eal_interrupts.c
index 5d0607effe..4cfaeba7fe 100644
--- a/lib/eal/linux/eal_interrupts.c
+++ b/lib/eal/linux/eal_interrupts.c
@@ -1443,7 +1443,7 @@ rte_epoll_ctl(int epfd, int op, int fd,
 
 	if (!event) {
 		EAL_LOG(ERR, "rte_epoll_event can't be NULL");
-		return -1;
+		return -EINVAL;
 	}
 
 	/* using per thread epoll fd */
@@ -1460,13 +1460,21 @@ rte_epoll_ctl(int epfd, int op, int fd,
 
 	ev.events = event->epdata.event;
 	if (epoll_ctl(epfd, op, fd, &ev) < 0) {
+		int err = errno;
+
+		/* the fd is already in the set (e.g. shared across vectors):
+		 * keep the event valid and report -EEXIST, not a hard error.
+		 */
+		if (op == EPOLL_CTL_ADD && err == EEXIST)
+			return -EEXIST;
+
 		EAL_LOG(ERR, "Error op %d fd %d epoll_ctl, %s",
-			op, fd, strerror(errno));
+			op, fd, strerror(err));
 		if (op == EPOLL_CTL_ADD)
 			/* rollback status when CTL_ADD fail */
 			rte_atomic_store_explicit(&event->status, RTE_EPOLL_INVALID,
 					rte_memory_order_relaxed);
-		return -1;
+		return -err;
 	}
 
 	if (op == EPOLL_CTL_DEL && rte_atomic_load_explicit(&event->status,
@@ -1518,8 +1526,6 @@ rte_intr_rx_ctl(struct rte_intr_handle *intr_handle, int epfd,
 			EAL_LOG(DEBUG,
 				"efd %d associated with vec %d added on epfd %d",
 				rev->fd, vec, epfd);
-		else
-			rc = -EPERM;
 		break;
 	case RTE_INTR_EVENT_DEL:
 		epfd_op = EPOLL_CTL_DEL;
@@ -1531,8 +1537,6 @@ rte_intr_rx_ctl(struct rte_intr_handle *intr_handle, int epfd,
 		}
 
 		rc = rte_epoll_ctl(rev->epfd, epfd_op, rev->fd, rev);
-		if (rc)
-			rc = -EPERM;
 		break;
 	default:
 		EAL_LOG(ERR, "event op type mismatch");
-- 
2.43.0