fs_poll: race condition protection when modifying revents value

pollfd revents value is occasionally zeroed by race condition when
poll_notify() is called. This results to false timeout return value
from poll().

This behaviour was found out when application called tcdrain() before
calling uart poll().

Signed-off-by: Tero Salminen <[email protected]>

fs/vfs/fs_poll.c

@@ -275,6 +275,7 @@ void poll_notify(FAR struct pollfd **afds, int nfds, pollevent_t eventset)
 {
   int i;
   FAR struct pollfd *fds;
+  irqstate_t flags;
 
   DEBUGASSERT(afds != NULL && nfds >= 1);
 
@@ -283,6 +284,9 @@ void poll_notify(FAR struct pollfd **afds, int nfds, pollevent_t eventset)
       fds = afds[i];
       if (fds != NULL)
         {
+          /* race condition protection when modifying fds->revents */
+          flags = enter_critical_section();
+
           /* The error event must be set in fds->revents */
 
           fds->revents |= eventset & (fds->events | POLLERR | POLLHUP);
@@ -293,6 +297,8 @@ void poll_notify(FAR struct pollfd **afds, int nfds, pollevent_t eventset)
               fds->revents &= ~POLLOUT;
             }
 
+          leave_critical_section(flags);
+
           if ((fds->revents != 0 || (fds->events & POLLALWAYS) != 0) &&
               fds->cb != NULL)
             {