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qemu-char: socket backend: disconnect on write error

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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 4bf1cb0).

Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
Signed-off-by: Denis 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: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Anton Nefedov 2017-02-02 17:26:29 +03:00 committed by Paolo Bonzini
parent a3fd46152e
commit b0a335e351
1 changed files with 10 additions and 0 deletions
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10
chardev/char-socket.c
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@ -97,6 +97,9 @@ static gboolean tcp_chr_accept(QIOChannel *chan,
GIOCondition cond, GIOCondition cond,
void *opaque); void *opaque);
static int tcp_chr_read_poll(void *opaque);
static void tcp_chr_disconnect(Chardev *chr);
/* Called with chr_write_lock held. */ /* Called with chr_write_lock held. */
static int tcp_chr_write(Chardev *chr, const uint8_t *buf, int len) 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; 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; return ret;
} else { } else {
/* XXX: indicate an error ? */ /* XXX: indicate an error ? */