issue: 4664603 fix rx ring fast-path pointer

The m_p_rx_ring pointer serves as a performance optimization for
single-ring sockets, avoiding map iteration overhead in the common
case. However, the logic to maintain this pointer was duplicated
across three locations with inconsistent behavior:

1. rx_add_ring_cb() - only set when map size == 1
2. rx_del_ring_cb() - conditionally nullified when removing
3. do_rings_migration_rx() - only set if previously NULL

This inconsistency caused a bug where m_p_rx_ring could point to a
stale ring after transitioning from single-ring to multi-ring
configurations, leading to incorrect CQ arming and packet loss in
multi-homed or bonded scenarios.

Introduce update_rx_ring_ptr() helper to centralize and standardize
the pointer maintenance logic:
- Sets m_p_rx_ring when exactly one ring exists (fast path)
- Nullifies m_p_rx_ring otherwise (forces map iteration)

Replace all three inconsistent code blocks with calls to this helper,
ensuring correct behavior during ring addition, removal, and migration.

Signed-off-by: Tomer Cabouly <[email protected]>

Author: tomerdbz

src/core/sock/sockinfo.cpp

@@ -1170,9 +1170,7 @@ void sockinfo::do_rings_migration_rx(resource_allocation_key &old_key)
         unlock_rx_q();
         m_rx_ring_map_lock.lock();
         lock_rx_q();
-        if (!m_p_rx_ring && m_rx_ring_map.size() == 1) {
-            m_p_rx_ring = m_rx_ring_map.begin()->first;
-        }
+        update_rx_ring_ptr();
         unlock_rx_q();
         m_rx_ring_map_lock.unlock();
 
@@ -1530,9 +1528,7 @@ void sockinfo::rx_add_ring_cb(ring *p_ring)
          *  - rx_del_ring_cb()
          *  - do_rings_migration_rx()
          */
-        if (m_rx_ring_map.size() == 1) {
-            m_p_rx_ring = m_rx_ring_map.begin()->first;
-        }
+        update_rx_ring_ptr();
 
         notify_epoll = true;
 
@@ -1620,11 +1616,7 @@ void sockinfo::rx_del_ring_cb(ring *p_ring)
             delete p_ring_info;
 
             if (m_p_rx_ring == base_ring) {
-                if (m_rx_ring_map.size() == 1) {
-                    m_p_rx_ring = m_rx_ring_map.begin()->first;
-                } else {
-                    m_p_rx_ring = nullptr;
-                }
+                update_rx_ring_ptr();
 
                 move_descs(base_ring, &temp_rx_reuse, &m_rx_reuse_buff.rx_reuse, true);
                 move_descs(base_ring, &temp_rx_reuse_global, &m_rx_reuse_buff.rx_reuse, false);
@@ -1828,6 +1820,15 @@ bool sockinfo::attach_as_uc_receiver_anyip(sa_family_t family, role_t role, bool
     return ret;
 }
 
+void sockinfo::update_rx_ring_ptr()
+{
+    if (m_rx_ring_map.size() == 1) {
+        m_p_rx_ring = m_rx_ring_map.begin()->first;
+    } else {
+        m_p_rx_ring = nullptr;
+    }
+}
+
 transport_t sockinfo::find_target_family(role_t role, const struct sockaddr *sock_addr_first,
                                          const struct sockaddr *sock_addr_second /* = NULL */)
 {

src/core/sock/sockinfo.h

@@ -444,6 +444,20 @@ class sockinfo {
     int fcntl_helper(int __cmd, unsigned long int __arg, bool &bexit);
     bool attach_as_uc_receiver_anyip(sa_family_t family, role_t role, bool skip_rules);
 
+    /**
+     * @brief Update the RX ring fast-path pointer based on ring map size.
+     *
+     * This method maintains the m_p_rx_ring optimization pointer for single-ring sockets.
+     * Performance optimization: When a socket has exactly one RX ring (the common case),
+     * m_p_rx_ring provides direct pointer access, avoiding map iteration overhead.
+     *
+     * Must be called after any operation that modifies m_rx_ring_map:
+     * - Adding rings (rx_add_ring_cb)
+     * - Removing rings (rx_del_ring_cb)
+     * - Ring migration (do_rings_migration_rx)
+     */
+    void update_rx_ring_ptr();
+
 protected:
     dst_entry *m_p_connected_dst_entry = nullptr;
     sockinfo_state m_state = SOCKINFO_OPENED; // socket current state