Re: POLLPRI/poll() behavior change since 2.6.31

Le jeudi 06 janvier 2011 à 13:50 -0200, Leonardo Chiquitto a écrit :

quoted

Hello,

Since 2.6.31, poll() no longer returns when waiting exclusively for a
POLLPRI event. If we wait for POLLPRI | POLLIN, though, it will
correctly return POLLPRI as a received event.

The reproducer (code below) will print the following when running on
2.6.30:

$ ./pollpri-oob
main: starting
main: setup_pipe ok (sfd[0] = 5, sfd[1] = 4)
parent: child <pid 3790> started
child: polling...
parent: sending the message
parent: waiting for child to exit
child: poll(): some data <1,2> has arrived!
child: done
parent: done

... and will block when running on 2.6.37-rc7:

$ ./pollpri-oob
main: starting
main: setup_pipe ok (sfd[0] = 5, sfd[1] = 4)
parent: child <pid 14148> started
child: polling...
parent: sending the message
parent: waiting for child to exit
[hangs here]

I've bisected this behavior change to the following commit:

commit 4938d7e0233a455f04507bac81d0886c71529537
Author: Eric Dumazet [off-list ref]
Date:   Tue Jun 16 15:33:36 2009 -0700

  poll: avoid extra wakeups in select/poll

  After introduction of keyed wakeups Davide Libenzi did on epoll, we are
  able to avoid spurious wakeups in poll()/select() code too.

  For example, typical use of poll()/select() is to wait for incoming
  network frames on many sockets.  But TX completion for UDP/TCP frames call
  sock_wfree() which in turn schedules thread.

  When scheduled, thread does a full scan of all polled fds and can sleep
  again, because nothing is really available.  If number of fds is large,
  this cause significant load.

  This patch makes select()/poll() aware of keyed wakeups and useless
  wakeups are avoided.  This reduces number of context switches by about 50%
  on some setups, and work performed by sofirq handlers.


I don't know if the new behavior is correct, but we've got one report of
an application that broke due to the change.

Hi Leonardo

Hmm, this is because    sock_def_readable() uses :

wake_up_interruptible_sync_poll(&wq->wait, POLLIN | POLLRDNORM |
POLLRDBAND);

So POLLPRI bit is not signaled.

I would just add POLLPRI flag in sock_def_readable()

(Alternatively, define a tcp_def_readable() function to pass POLLPRI
only if TCP_URG is set, but is it worth the pain for a seldom used
feature ?)

David, do you have an opinion on this ?

Thanks

diff --git a/net/core/sock.c b/net/core/sock.c
index e5af8d5..7fd3541 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c

@@ -1907,7 +1907,7 @@ static void sock_def_readable(struct sock *sk, int len)

       rcu_read_lock();
       wq = rcu_dereference(sk->sk_wq);
       if (wq_has_sleeper(wq))
-               wake_up_interruptible_sync_poll(&wq->wait, POLLIN |
+               wake_up_interruptible_sync_poll(&wq->wait, POLLIN | POLLPRI |
                                               POLLRDNORM | POLLRDBAND);
       sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_IN);
       rcu_read_unlock();