bdbuf: Fix race condition with sync active flag

Bug report by Oleg Kravtsov:

In rtems_bdbuf_swapout_processing() function there is the following
lines:

if (bdbuf_cache.sync_active && !transfered_buffers)
{

    rtems_id sync_requester;
    rtems_bdbuf_lock_cache ();
    ...

}

Here access to bdbuf_cache.sync_active is not protected with anything.
Imagine the following test case:

1. Task1 releases buffer(s) with bdbuf_release_modified() calls;

2. After a while swapout task starts and flushes all buffers;

3. In the end of that swapout flush we are before that part of code, and
assume there is task switching (just before "if (bdbuf_cache.sync_active
&& !transfered_buffers)");

4. Some other task (with higher priority) does bdbuf_release_modified
and rtems_bdbuf_syncdev().

This task successfully gets both locks sync and pool (in
rtems_bdbuf_syncdev() function), sets sync_active to true and starts
waiting for RTEMS_BDBUF_TRANSFER_SYNC event with only sync lock got.

5. Task switching happens again and we are again before "if
(bdbuf_cache.sync_active && !transfered_buffers)".

As the result we check sync_active and we come inside that "if"
statement.

6. The result is that we send RTEMS_BDBUF_TRANSFER_SYNC event! Though
ALL modified messages of that task are not flushed yet!

close #1485

cpukit/libblock/src/bdbuf.c

@@ -2534,9 +2534,15 @@ rtems_bdbuf_swapout_processing (unsigned long                 timer_delta,
 {
   rtems_bdbuf_swapout_worker* worker;
   bool                        transfered_buffers = false;
+  bool                        sync_active;
 
   rtems_bdbuf_lock_cache ();
 
+  /*
+   * To set this to true you need the cache and the sync lock.
+   */
+  sync_active = bdbuf_cache.sync_active;
+
   /*
    * If a sync is active do not use a worker because the current code does not
    * cleaning up after. We need to know the buffers have been written when
@@ -2546,7 +2552,7 @@ rtems_bdbuf_swapout_processing (unsigned long                 timer_delta,
    * lock. The simplest solution is to get the main swap out task perform all
    * sync operations.
    */
-  if (bdbuf_cache.sync_active)
+  if (sync_active)
     worker = NULL;
   else
   {
@@ -2558,16 +2564,16 @@ rtems_bdbuf_swapout_processing (unsigned long                 timer_delta,
 
   rtems_chain_initialize_empty (&transfer->bds);
   transfer->dev = BDBUF_INVALID_DEV;
-  transfer->syncing = bdbuf_cache.sync_active;
+  transfer->syncing = sync_active;
-  
+
   /*
    * When the sync is for a device limit the sync to that device. If the sync
    * is for a buffer handle process the devices in the order on the sync
    * list. This means the dev is BDBUF_INVALID_DEV.
    */
-  if (bdbuf_cache.sync_active)
+  if (sync_active)
     transfer->dev = bdbuf_cache.sync_device;
-    
+
   /*
    * If we have any buffers in the sync queue move them to the modified
    * list. The first sync buffer will select the device we use.
@@ -2584,7 +2590,7 @@ rtems_bdbuf_swapout_processing (unsigned long                 timer_delta,
   rtems_bdbuf_swapout_modified_processing (&transfer->dev,
                                            &bdbuf_cache.modified,
                                            &transfer->bds,
-                                           bdbuf_cache.sync_active,
+                                           sync_active,
                                            update_timers,
                                            timer_delta);
 
@@ -2615,7 +2621,7 @@ rtems_bdbuf_swapout_processing (unsigned long                 timer_delta,
     transfered_buffers = true;
   }
 
-  if (bdbuf_cache.sync_active && !transfered_buffers)
+  if (sync_active && !transfered_buffers)
   {
     rtems_id sync_requester;
     rtems_bdbuf_lock_cache ();