Fix a semphore overflow problem in the CAN driver

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@3890 42af7a65-404d-4744-a932-0658087f49c3

ChangeLog

@@ -2002,3 +2002,6 @@
 	  and bit fields within all Kinetis registers.
 	* configs/twr-k60n512:  Add support for the Kinetis K60 Tower board
 	  (TWR-K60N512).
+	* drivers/can.c:  Fixe a semaphore overflow problem in the CAN driver
+	  (reported by Li Zhouy (Lzzy)).
+

drivers/can.c

@@ -50,6 +50,7 @@
 #include <string.h>
 #include <semaphore.h>
 #include <fcntl.h>
+#include <assert.h>
 #include <errno.h>
 #include <debug.h>
 
@@ -443,7 +444,11 @@ static ssize_t can_write(FAR struct file *filep, FAR const char *buffer, size_t
 
           do
             {
+              DEBUGASSERT(dev->cd_ntxwaiters < 255);
+              dev->cd_ntxwaiters++;
               ret = sem_wait(&fifo->cf_sem);
+              dev->cd_ntxwaiters--;
+
               if (ret < 0 && errno != EINTR)
                 {
                   ret = -errno;
@@ -750,9 +755,12 @@ int can_txdone(FAR struct can_dev_s *dev)
       /* Send the next message in the FIFO */
 
       ret = can_xmit(dev);
-      if (ret == OK)
+
+      /* Are there any threads waiting for space in the TX FIFO? */
+
+      if (ret == OK && dev->cd_ntxwaiters > 0)
         {
-          /* Inform any waiting threads that new xmit space is available */
+          /* Yes.. Inform them that new xmit space is available */
 
           ret = sem_post(&dev->cd_xmit.cf_sem);
         }

include/nuttx/can.h

@@ -219,6 +219,7 @@ struct can_dev_s
 {
   uint8_t              cd_ocount;        /* The number of times the device has been opened */
   uint8_t              cd_npendrtr;      /* Number of pending RTR messages */
+  uint8_t              cd_ntxwaiters;    /* Number of threads waiting to enqueue a message */
   sem_t                cd_closesem;      /* Locks out new opens while close is in progress */
   sem_t                cd_recvsem;       /* Used to wakeup user waiting for space in cd_recv.buffer */
   struct can_fifo_s    cd_xmit;          /* Describes transmit FIFO */