[PATCH v3] net/mlx5: fix counter TAILQ race between free and query callback

[Linhu Li <lilinhu618@gmail.com>]

flow_dv_counter_free() inserts counters into
pool->counters[pool->query_gen] under pool->csl. Meanwhile,
mlx5_flow_async_pool_query_handle() moves counters from
pool->counters[query_gen ^ 1] to the global free list via
TAILQ_CONCAT while holding only cmng->csl, not pool->csl.

The comment in flow_dv_counter_free() claims the lock is not needed
because the query callback and the release function operate on different lists.
That holds only if the free path always observes the up-to-date query_gen. It
can be violated:

1. A counter free thread (non-PMD, e.g. OVS offload thread) reads
   pool->query_gen == 0 and is about to insert into counters[0].
2. The free thread is preempted by the OS scheduler; it is a regular
   pthread, not pinned to a core.
3. The eal-intr-thread alarm fires: query_gen++ (now 1) and the async
   query is sent.
4. Hardware completes the query and the callback runs TAILQ_CONCAT on
   counters[0] (= query_gen ^ 1).
5. The free thread resumes and runs TAILQ_INSERT_TAIL on counters[0]
   concurrently with step 4 on another core.

Because the two paths take different locks, TAILQ_INSERT_TAIL and
TAILQ_CONCAT run concurrently on the same list with no synchronization and
corrupt it: the pool-local list ends up with a NULL head but a dangling
tqh_last, and the global free list tail no longer points to the real tail. The just-
freed counter and every counter inserted afterwards become unreachable
and are leaked.

Non-PMD threads can be preempted for hundreds of microseconds under
CPU pressure, which is well within the async query round-trip time, so the
window is reachable in practice.

Fix it by taking pool->csl in the query completion callback before operating on
pool->counters[query_gen], serializing the CONCAT with any concurrent
INSERT. The lock is taken once per pool per query completion in the eal-intr-
thread context, not on the datapath, so the cost is negligible. Lock order is
pool->csl then cmng->csl, matching all other sites.

Also handle the error path: previously the counters accumulated in
pool->counters[query_gen] were abandoned when a query failed. Move
them back to the global free list to avoid a leak on persistent query failures.

Fixes: ac79183dc6f7 ("net/mlx5: optimize free counter lookup")
Cc: stable@dpdk.org

Signed-off-by: Linhu Li <lilinhu618@gmail.com>
Acked-by: Dariusz Sosnowski <dsosnowski@nvidia.com>
---
 doc/guides/rel_notes/release_26_07.rst | 21 +++++++++++++++++
 drivers/net/mlx5/mlx5_flow.c           | 31 ++++++++++++++++++++++++++
 2 files changed, 52 insertions(+)

diff --git a/doc/guides/rel_notes/release_26_07.rst b/doc/guides/rel_notes/release_26_07.rst
index b8a3e2ced9..30a9564884 100644
--- a/doc/guides/rel_notes/release_26_07.rst
+++ b/doc/guides/rel_notes/release_26_07.rst
@@ -153,6 +153,27 @@ ABI Changes
 * No ABI change that would break compatibility with 25.11.
 
 
+Fixed Issues
+------------
+
+.. This section should contain fixed issues in this release. Sample format:
+
+   * **Add a title in the past tense with a full stop.**
+
+     Add a short 1-2 sentence description of the fix in the past tense.
+
+   This section is a comment. Do not overwrite or remove it.
+   Also, make sure to start the actual text at the margin.
+   =======================================================
+
+* **net/mlx5: Fixed counter TAILQ race between free and query callback.**
+
+  Fixed a race condition where concurrent counter free operations and async
+  query completions could corrupt the counter free list, causing counter leaks.
+  The issue occurred when non-PMD threads were preempted between reading
+  ``query_gen`` and inserting into the counter list.
+
+
 Known Issues
 ------------
 
diff --git a/drivers/net/mlx5/mlx5_flow.c b/drivers/net/mlx5/mlx5_flow.c
index 915ea29a5a..2f785d58ec 100644
--- a/drivers/net/mlx5/mlx5_flow.c
+++ b/drivers/net/mlx5/mlx5_flow.c
@@ -9893,6 +9893,13 @@ void
 mlx5_flow_async_pool_query_handle(struct mlx5_dev_ctx_shared *sh,
 				  uint64_t async_id, int status)
 {
+	/*
+	 * Handle async counter pool query completion.
+	 * query_gen is flipped each round: freed counters go into [query_gen],
+	 * while this callback moves [query_gen ^ 1] to the global free list.
+	 * pool->csl must be held when operating on pool->counters[] to serialize
+	 * with concurrent free-path insertions.
+	 */
 	struct mlx5_flow_counter_pool *pool =
 		(struct mlx5_flow_counter_pool *)(uintptr_t)async_id;
 	struct mlx5_counter_stats_raw *raw_to_free;
@@ -9904,6 +9911,21 @@ mlx5_flow_async_pool_query_handle(struct mlx5_dev_ctx_shared *sh,
 
 	if (unlikely(status)) {
 		raw_to_free = pool->raw_hw;
+		/*
+		 * The query failed, so the freed counters accumulated
+		 * in the old-gen list would otherwise be stranded.
+		 * Move them back to the global free list. This is safe
+		 * for both transient and persistent failures: the
+		 * counters are still valid and can be reused.
+		 */
+		if (!TAILQ_EMPTY(&pool->counters[query_gen])) {
+			rte_spinlock_lock(&pool->csl);
+			rte_spinlock_lock(&cmng->csl[cnt_type]);
+			TAILQ_CONCAT(&cmng->counters[cnt_type],
+				     &pool->counters[query_gen], next);
+			rte_spinlock_unlock(&cmng->csl[cnt_type]);
+			rte_spinlock_unlock(&pool->csl);
+		}
 	} else {
 		raw_to_free = pool->raw;
 		if (pool->is_aged)
@@ -9913,11 +9935,20 @@ mlx5_flow_async_pool_query_handle(struct mlx5_dev_ctx_shared *sh,
 		rte_spinlock_unlock(&pool->sl);
 		/* Be sure the new raw counters data is updated in memory. */
 		rte_io_wmb();
+		/*
+		 * A counter free thread may have read a stale query_gen
+		 * before the generation was flipped and could still be
+		 * inserting into this same old-gen list. Hold pool->csl to
+		 * serialize TAILQ_CONCAT with that TAILQ_INSERT_TAIL and
+		 * avoid corrupting the list.
+		 */
 		if (!TAILQ_EMPTY(&pool->counters[query_gen])) {
+			rte_spinlock_lock(&pool->csl);
 			rte_spinlock_lock(&cmng->csl[cnt_type]);
 			TAILQ_CONCAT(&cmng->counters[cnt_type],
 				     &pool->counters[query_gen], next);
 			rte_spinlock_unlock(&cmng->csl[cnt_type]);
+			rte_spinlock_unlock(&pool->csl);
 		}
 	}
 	LIST_INSERT_HEAD(&sh->sws_cmng.free_stat_raws, raw_to_free, next);
-- 
2.39.3 (Apple Git-146)