Commit b0a335e3 authored by Anton Nefedov's avatar Anton Nefedov Committed by Paolo Bonzini
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qemu-char: socket backend: disconnect on write error 



Socket backend read handler should normally perform a disconnect, however
the read handler may not get a chance to run if the frontend is not ready
(qemu_chr_be_can_write() == 0).

This means that in virtio-serial frontend case if
 - the host has disconnected (giving EPIPE on socket write)
 - and the guest has disconnected (-> frontend not ready -> backend
   will not read)
 - and there is still data (frontend->backend) to flush (has to be a really
   tricky timing but nevertheless, we have observed the case in production)

This results in virtio-serial trying to flush this data continiously forming
a busy loop.

Solution: react on write error in the socket write handler.
errno is not reliable after qio_channel_writev_full(), so we may not get
the exact EPIPE, so disconnect on any error but QIO_CHANNEL_ERR_BLOCK which
io_channel_send_full() converts to errno EAGAIN.
We must not disconnect right away though, there still may be data to read
(see 4bf1cb03).

Signed-off-by: default avatarAnton Nefedov <anton.nefedov@virtuozzo.com>
Signed-off-by: default avatarDenis V. Lunev <den@openvz.org>
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Daniel P. Berrange <berrange@redhat.com>
CC: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1486045589-8074-1-git-send-email-den@openvz.org>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent a3fd4615

chardev/char-socket.c

+10 −0
Original line number Diff line number Diff line
@@ -97,6 +97,9 @@ static gboolean tcp_chr_accept(QIOChannel *chan, 
                               GIOCondition cond,

                               void *opaque);



static int tcp_chr_read_poll(void *opaque);

static void tcp_chr_disconnect(Chardev *chr);



/* Called with chr_write_lock held.  */

static int tcp_chr_write(Chardev *chr, const uint8_t *buf, int len)

{
@@ -114,6 +117,13 @@ static int tcp_chr_write(Chardev *chr, const uint8_t *buf, int len) 
            s->write_msgfds_num = 0;

        }



        if (ret < 0 && errno != EAGAIN) {

            if (tcp_chr_read_poll(chr) <= 0) {

                tcp_chr_disconnect(chr);

                return len;

            } /* else let the read handler finish it properly */

        }



        return ret;

    } else {

        /* XXX: indicate an error ? */