[PATCH v1 2/5] eal: fix async IPC callback not fired when no peers

[Anatoly Burakov]

Currently, when rte_mp_request_async() is called and no peer processes
are connected (nb_sent == 0), the user callback is never invoked.

The original implementation used a dedicated background thread and
pthread_cond_signal() to wake it after queuing the dummy request. When
that thread was replaced with per-message alarms, no alarm was set for
the dummy request, silently breaking the nb_sent == 0 path.

This was not noticed because async requests are used while handling
secondary process requests, where peers are typically already present.

Fix it by setting a 1us alarm on the dummy request, so the callback path
immediately triggers and processes it.

Fixes: daf9bfca717e ("ipc: remove thread for async requests")
Cc: stable at dpdk.org

Signed-off-by: Anatoly Burakov <anatoly.burakov at intel.com>
---
 lib/eal/common/eal_common_proc.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/lib/eal/common/eal_common_proc.c b/lib/eal/common/eal_common_proc.c
index 799c6e81b0..0ec79336a5 100644
--- a/lib/eal/common/eal_common_proc.c
+++ b/lib/eal/common/eal_common_proc.c
@@ -1187,11 +1187,15 @@ rte_mp_request_async(struct rte_mp_msg *req, const struct timespec *ts,
 	if (rte_eal_process_type() == RTE_PROC_SECONDARY) {
 		ret = mp_request_async(eal_mp_socket_path(), copy, param, ts);
 
-		/* if we didn't send anything, put dummy request on the queue */
+		/* if we didn't send anything, put dummy request on the queue
+		 * and set a minimum-delay alarm so the callback fires immediately.
+		 */
 		if (ret == 0 && reply->nb_sent == 0) {
 			TAILQ_INSERT_TAIL(&pending_requests.requests, dummy,
 					next);
 			dummy_used = true;
+			if (rte_eal_alarm_set(1, async_reply_handle, dummy) < 0)
+				EAL_LOG(ERR, "Fail to set alarm for dummy request");
 		}
 
 		pthread_mutex_unlock(&pending_requests.lock);
@@ -1232,10 +1236,14 @@ rte_mp_request_async(struct rte_mp_msg *req, const struct timespec *ts,
 		} else if (mp_request_async(path, copy, param, ts))
 			ret = -1;
 	}
-	/* if we didn't send anything, put dummy request on the queue */
+	/* if we didn't send anything, put dummy request on the queue
+	 * and set a minimum-delay alarm so the callback fires immediately.
+	 */
 	if (ret == 0 && reply->nb_sent == 0) {
 		TAILQ_INSERT_HEAD(&pending_requests.requests, dummy, next);
 		dummy_used = true;
+		if (rte_eal_alarm_set(1, async_reply_handle, dummy) < 0)
+			EAL_LOG(ERR, "Fail to set alarm for dummy request");
 	}
 
 	/* finally, unlock the queue */
-- 
2.47.3