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[Qemu-devel] [PULL 05/23] qemu-char: socket backend: disconnect on write

From: Paolo Bonzini
Subject: [Qemu-devel] [PULL 05/23] qemu-char: socket backend: disconnect on write error
Date: Thu, 16 Feb 2017 15:31:29 +0100

From: Anton Nefedov <address@hidden>

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 <address@hidden>
Signed-off-by: Denis V. Lunev <address@hidden>
CC: Paolo Bonzini <address@hidden>
CC: Daniel P. Berrange <address@hidden>
CC: Marc-André Lureau <address@hidden>
Message-Id: <address@hidden>
Signed-off-by: Paolo Bonzini <address@hidden>
 chardev/char-socket.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/chardev/char-socket.c b/chardev/char-socket.c
index 4068dc5..865c527 100644
--- a/chardev/char-socket.c
+++ b/chardev/char-socket.c
@@ -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 ? */

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