net/tcp/tcp_send_buffered.c: Fix deadlock in iob_copyin when iob buffers are exhausted and network lock is taken.

configs/sabre-6quad/README.txt

@@ -4,6 +4,12 @@ README.txt
 This directory holds a port of NuttX to the NXP/Freescale Sabre board
 featuring the iMX 6Quad CPU.
 
+This is a minimal port, used primarily for verifying SMP operation.  More
+recently, a port to the i.MX RT was added.  This port has gotten more
+support since it is better aligned with usage in embedded systems.  The
+i.MX6 and the i.MX6 share IOMUXing and some peripherals.  It ought to be
+a simple matter to backport some of the common drivers from i.MXRT to i.MX6.
+
 Contents
 ========
 

net/tcp/tcp_send_buffered.c

@@ -68,6 +68,7 @@
 #include <nuttx/net/netdev.h>
 #include <nuttx/net/arp.h>
 #include <nuttx/net/tcp.h>
+#include <nuttx/net/net.h>
 
 #include "netdev/netdev.h"
 #include "devif/devif.h"
@@ -1175,7 +1176,20 @@ ssize_t psock_tcp_send(FAR struct socket *psock, FAR const void *buf,
         }
       else
         {
+          int count;
+          int blresult;
+
+          /* iob_copyin might wait for buffers to be freed, but if network is
+           * locked this might never happen, since network driver is also locked,
+           * therefore we need to break the lock
+           */
+
+          blresult = net_breaklock(&count);
           result = TCP_WBCOPYIN(wrb, (FAR uint8_t *)buf, len);
+          if (blresult >= 0)
+            {
+              net_restorelock(count);
+            }
         }
 
       /* Dump I/O buffer chain */