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

0 files changed, 0 insertions, 0 deletions