[pulseaudio-discuss] Race condition in pa_poll (was: Bug in pa_poll?)

[Marcel Müller]

It semms that my suggestion will still not always fix the problem.
There is still an ABA problem, Because the descriptor that just died 
could already be reused for a different purpose. So I changed the code 
to retry several times until it fails. This fixed the deadlocks so far.

Unfortunately the native OS/2 select implementation has a different 
interface. So the code is not 1:1 portable.


On 17.03.12 10.13, Marcel Müller wrote:
> If select() in the implementation of pa_poll returns an error because of
> an invalid socket, pa_poll tries to find out which descriptor is bad.
> And then retries the select. But this is still in not synchronized
> context. So a second descriptor may become bad on the second call to
> select. This is not captured. And consequently pa_poll fails and the
> mainloop terminates, resulting in deadlocks if further activity is on
> the way.
> It seems that I managed to trigger this problem on OS/2 from time to
> time when closing a pa_stream and a pa_context and opening another
> context at almost the same time.
>
> I would recommend to jump to the previous select() call instead of
> executing another one without error checking. To avoid infinite loops
> this should only be done if the check removed at least one descriptor.


-----
#if defined(OS_IS_OS2)
int pa_poll (struct pollfd *fds, unsigned long int nfds, int timeout) {
     int* selectfds;
     int nselectfds;
     struct pollfd* f;
     struct pollfd* fdsend = fds + nfds;
     int nreads, nwrites, nexcepts;
     int* pselectfd;
     int ready;
     int nretry = nfds;

     /* First check how many descriptors we need */
     nselectfds = 0;
     for (f = fds; f != fdsend; ++f)
     {   int events = f->events;
         nselectfds += !!(events & POLLIN)
                     + !!(events & POLLPRI)
                     + !!(events & POLLOUT);
         f->revents = 0;
     }
     selectfds = alloca(nselectfds * sizeof *selectfds);

   retry:
     /* populate selectfds */
     nreads = 0;
     nwrites = 0;
     nexcepts = 0;
     pselectfd = selectfds;
     for (f = fds; f != fdsend; ++f)
     {   if (f->events & POLLIN)
         {   *pselectfd++ = f->fd;
             ++nreads;
         }
     }
     for (f = fds; f != fdsend; ++f)
     {   if (f->events & POLLOUT)
         {   *pselectfd++ = f->fd;
             ++nwrites;
         }
     }
     for (f = fds; f != fdsend; ++f)
     {   if (f->events & POLLPRI)
         {   *pselectfd++ = f->fd;
             ++nexcepts;
         }
     }

     /* execute select */
     ready = os2_select(selectfds, nreads, nwrites, nexcepts, timeout);

     if (ready == -1)
     {   errno = sock_errno();
         /* One of the descriptors is bad. Find out which one. */
         int removed = 0;
         for (f = fds; f != fdsend; ++f)
             if (f->events != 0)
             {   if (os2_select(&f->fd, 1, 0, 0, 0) == -1)
                 {   /* do not care any more */
                     f->events = 0;
                     ++removed;
                 }
             }
         DEBUGLOG(("pa_poll - recover? %i\n", removed));
         if (nretry)
         {   --nretry;
             goto retry;
         }
     } else if (ready)
     {   /* Find out the descriptors that got ready. */
         ready = 0;
         pselectfd = selectfds;
         for (f = fds; f != fdsend; ++f)
         {   if (f->events & POLLIN)
             {   if (*pselectfd++ != -1)
                 {   ++ready;
                     f->revents = POLLIN;
                 }
             }
         }
         for (f = fds; f != fdsend; ++f)
         {   if (f->events & POLLOUT)
             {   if (*pselectfd++ != -1)
                 {   if (!f->revents)
                         ++ready;
                     f->revents |= POLLOUT;
                 }
             }
         }
         for (f = fds; f != fdsend; ++f)
         {   if (f->events & POLLPRI)
             {   if (*pselectfd++ != -1)
                 {   if (!f->revents)
                         ++ready;
                     f->revents |= POLLPRI;
                 }
             }
         }
     }
     return ready;
}

#elif !defined(HAVE_POLL_H) || defined(OS_IS_DARWIN)