From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:33799) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gx9lK-0003Q9-CG for qemu-devel@nongnu.org; Fri, 22 Feb 2019 07:17:09 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gx9lJ-0003ca-CI for qemu-devel@nongnu.org; Fri, 22 Feb 2019 07:17:06 -0500 Received: from mx1.redhat.com ([209.132.183.28]:60812) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gx9lI-0003aQ-Vk for qemu-devel@nongnu.org; Fri, 22 Feb 2019 07:17:05 -0500 Date: Fri, 22 Feb 2019 12:16:57 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Message-ID: <20190222121657.GP25234@redhat.com> Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Subject: Re: [Qemu-devel] [PATCH v2 3/3] char-socket: Lock tcp_chr_disconnect() and socket_reconnect_timeout() List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Alberto Garcia Cc: qemu-devel@nongnu.org, Paolo Bonzini , =?utf-8?Q?Marc-Andr=C3=A9?= Lureau On Fri, Feb 22, 2019 at 01:59:12PM +0200, Alberto Garcia wrote: > There's a race condition in which the tcp_chr_read() ioc handler can > close a connection that is being written to from another thread. > > Running iotest 136 in a loop triggers this problem and crashes QEMU. > > (gdb) bt > #0 0x00005558b842902d in object_get_class (obj=0x0) at qom/object.c:860 > #1 0x00005558b84f92db in qio_channel_writev_full (ioc=0x0, iov=0x7ffc355decf0, niov=1, fds=0x0, nfds=0, errp=0x0) at io/channel.c:76 > #2 0x00005558b84e0e9e in io_channel_send_full (ioc=0x0, buf=0x5558baf5beb0, len=138, fds=0x0, nfds=0) at chardev/char-io.c:123 > #3 0x00005558b84e4a69 in tcp_chr_write (chr=0x5558ba460380, buf=0x5558baf5beb0 "...", len=138) at chardev/char-socket.c:135 > #4 0x00005558b84dca55 in qemu_chr_write_buffer (s=0x5558ba460380, buf=0x5558baf5beb0 "...", len=138, offset=0x7ffc355dedd0, write_all=false) at chardev/char.c:112 > #5 0x00005558b84dcbc2 in qemu_chr_write (s=0x5558ba460380, buf=0x5558baf5beb0 "...", len=138, write_all=false) at chardev/char.c:147 > #6 0x00005558b84dfb26 in qemu_chr_fe_write (be=0x5558ba476610, buf=0x5558baf5beb0 "...", len=138) at chardev/char-fe.c:42 > #7 0x00005558b8088c86 in monitor_flush_locked (mon=0x5558ba476610) at monitor.c:406 > #8 0x00005558b8088e8c in monitor_puts (mon=0x5558ba476610, str=0x5558ba921e49 "") at monitor.c:449 > #9 0x00005558b8089178 in qmp_send_response (mon=0x5558ba476610, rsp=0x5558bb161600) at monitor.c:498 > #10 0x00005558b808920c in monitor_qapi_event_emit (event=QAPI_EVENT_SHUTDOWN, qdict=0x5558bb161600) at monitor.c:526 > #11 0x00005558b8089307 in monitor_qapi_event_queue_no_reenter (event=QAPI_EVENT_SHUTDOWN, qdict=0x5558bb161600) at monitor.c:551 > #12 0x00005558b80896c0 in qapi_event_emit (event=QAPI_EVENT_SHUTDOWN, qdict=0x5558bb161600) at monitor.c:626 > #13 0x00005558b855f23b in qapi_event_send_shutdown (guest=false, reason=SHUTDOWN_CAUSE_HOST_QMP_QUIT) at qapi/qapi-events-run-state.c:43 > #14 0x00005558b81911ef in qemu_system_shutdown (cause=SHUTDOWN_CAUSE_HOST_QMP_QUIT) at vl.c:1837 > #15 0x00005558b8191308 in main_loop_should_exit () at vl.c:1885 > #16 0x00005558b819140d in main_loop () at vl.c:1924 > #17 0x00005558b8198c84 in main (argc=18, argv=0x7ffc355df3f8, envp=0x7ffc355df490) at vl.c:4665 > > This patch adds a lock to protect tcp_chr_disconnect() and > socket_reconnect_timeout() Can you think of any way to test this in the unit tests ? I can understand if its too difficult but just curious if there's any viable option ? > diff --git a/chardev/char-socket.c b/chardev/char-socket.c > index 4fcdd8aedd..67c2eeac6d 100644 > --- a/chardev/char-socket.c > +++ b/chardev/char-socket.c > @@ -172,7 +172,9 @@ static int tcp_chr_write(Chardev *chr, const uint8_t *buf, int len) > > if (ret < 0 && errno != EAGAIN) { > if (tcp_chr_read_poll(chr) <= 0) { > + qemu_mutex_unlock(&chr->chr_write_lock); > tcp_chr_disconnect(chr); > + qemu_mutex_lock(&chr->chr_write_lock); The idea of unlock & relocking here, just so tcp_chr_disconnect can also acquire locks, doesn't make me too happy. This is really an anti-pattern to me. I think we should rename the existing method to be tcp_chr_disconnect_locked() and document that it must only be called with the write lock held, and call that from here. Then introduce a new wrapper for all the other callers to carry on using which does the locking static void tcp_chr_disconnect(Chardev *chr) { qemu_mutex_lock(&chr->chr_write_lock); tcp_chr_disconnect_locked(chr); qemu_mutex_unlock(&chr->chr_write_lock); } Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|