Improvements in TCP connections allocation.

net/tcp/Kconfig

@@ -50,11 +50,46 @@ config NET_TCPURGDATA
 		compiled in. Urgent data (out-of-band data) is a rarely used TCP feature
 		that is very seldom would be required.
 
-config NET_TCP_CONNS
-	int "Number of TCP/IP connections"
+config NET_TCP_PREALLOC_CONNS
+	int "Preallocated TCP/IP connections"
 	default 8
 	---help---
-		Maximum number of TCP/IP connections (all tasks)
+		Number of TCP/IP connections (all tasks).
+		
+		This number of connections will be pre-allocated during system boot.
+		If dynamic connections allocation is enabled, more connections may
+		be allocated at a later time, as the system needs them. Else this
+		will be the maximum number of connections available to the system
+		at all times.
+
+		Set to 0 to disable (and rely only on dynamic allocations).
+
+config NET_TCP_ALLOC_CONNS
+	int "Dynamic TCP/IP connections allocation"
+	default 0
+	---help---
+		Dynamic memory allocations for TCP/IP.
+
+		When set to 0 all dynamic allocations are disabled.
+
+		When set to 1 a new connection will be allocated every time,
+		and it will be free'd when no longer needed.
+
+		Setting this to 2 or more will allocate the connections in
+		batches (with batch size equal to this config). When a
+		connection is no longer needed, it will be returned to the
+		free connections pool, and it will never be deallocated!
+
+config NET_TCP_MAX_CONNS
+	int "Maximum number of TCP/IP connections"
+	default 0
+	depends on NET_TCP_ALLOC_CONNS > 0
+	---help---
+		If dynamic connections allocation is selected (NET_TCP_ALLOC_CONNS > 0)
+		this will limit the number of connections that can be allocated.
+		
+		This is useful in case the system is under very heavy load (or
+		under attack), ensuring that the heap will not be exhausted.
 
 config NET_TCP_NPOLLWAITERS
 	int "Number of TCP poll waiters"

net/tcp/tcp_conn.c

@@ -76,8 +76,8 @@
 
 /* The array containing all TCP connections. */
 
-#ifndef CONFIG_NET_ALLOC_CONNS
-static struct tcp_conn_s g_tcp_connections[CONFIG_NET_TCP_CONNS];
+#if CONFIG_NET_TCP_PREALLOC_CONNS > 0
+static struct tcp_conn_s g_tcp_connections[CONFIG_NET_TCP_PREALLOC_CONNS];
 #endif
 
 /* A list of all free TCP connections */
@@ -439,7 +439,7 @@ static inline int tcp_ipv6_bind(FAR struct tcp_conn_s *conn,
  *
  ****************************************************************************/
 
-#ifdef CONFIG_NET_ALLOC_CONNS
+#if CONFIG_NET_TCP_ALLOC_CONNS > 0
 FAR struct tcp_conn_s *tcp_alloc_conn(void)
 {
   FAR struct tcp_conn_s *conn;
@@ -449,8 +449,16 @@ FAR struct tcp_conn_s *tcp_alloc_conn(void)
 
   if (dq_peek(&g_free_tcp_connections) == NULL)
     {
+#if CONFIG_NET_TCP_MAX_CONNS > 0
+      if (dq_count(&g_active_tcp_connections) + CONFIG_NET_TCP_ALLOC_CONNS
+          >= CONFIG_NET_TCP_MAX_CONNS)
+        {
+          return NULL;
+        }
+#endif
+
       conn = kmm_zalloc(sizeof(struct tcp_conn_s) *
-                        CONFIG_NET_TCP_CONNS);
+                        CONFIG_NET_TCP_ALLOC_CONNS);
       if (conn == NULL)
         {
           return conn;
@@ -458,7 +466,7 @@ FAR struct tcp_conn_s *tcp_alloc_conn(void)
 
       /* Now initialize each connection structure */
 
-      for (i = 0; i < CONFIG_NET_TCP_CONNS; i++)
+      for (i = 0; i < CONFIG_NET_TCP_ALLOC_CONNS; i++)
         {
           /* Mark the connection closed and move it to the free list */
 
@@ -597,10 +605,10 @@ int tcp_selectport(uint8_t domain,
 
 void tcp_initialize(void)
 {
-#ifndef CONFIG_NET_ALLOC_CONNS
+#if CONFIG_NET_TCP_PREALLOC_CONNS > 0
   int i;
 
-  for (i = 0; i < CONFIG_NET_TCP_CONNS; i++)
+  for (i = 0; i < CONFIG_NET_TCP_PREALLOC_CONNS; i++)
     {
       /* Mark the connection closed and move it to the free list */
 
@@ -700,7 +708,12 @@ FAR struct tcp_conn_s *tcp_alloc(uint8_t domain)
 
           tcp_free(conn);
 
-          /* Now there is guaranteed to be one free connection.  Get it! */
+          /* Now there should be one free connection. If dynamic connections
+           * allocation is disabled, it is guaranteed so. In case that
+           * dynamic connections are used, it may be already in the free
+           * list, or at least there should be enough space in the heap for
+           * a new connection.
+           */
 
           conn = (FAR struct tcp_conn_s *)
             dq_remfirst(&g_free_tcp_connections);
@@ -710,7 +723,7 @@ FAR struct tcp_conn_s *tcp_alloc(uint8_t domain)
 
   /* Allocate the connect entry from heap */
 
-#ifdef CONFIG_NET_ALLOC_CONNS
+#if CONFIG_NET_TCP_ALLOC_CONNS > 0
   if (conn == NULL)
     {
       conn = tcp_alloc_conn();
@@ -876,10 +889,26 @@ void tcp_free(FAR struct tcp_conn_s *conn)
     }
 #endif
 
-  /* Mark the connection available and put it into the free list */
+  /* Mark the connection available. */
 
   conn->tcpstateflags = TCP_CLOSED;
-  dq_addlast(&conn->sconn.node, &g_free_tcp_connections);
+
+  /* If this is a preallocated or a batch allocated connection store it in
+   * the free connections list. Else free it.
+   */
+
+#if CONFIG_NET_TCP_ALLOC_CONNS == 1
+  if (conn < g_tcp_connections || conn >= (g_tcp_connections +
+      CONFIG_NET_TCP_PREALLOC_CONNS))
+    {
+      kmm_free(conn);
+    }
+  else
+#endif
+    {
+      dq_addlast(&conn->sconn.node, &g_free_tcp_connections);
+    }
+
   net_unlock();
 }