I've got a case where usbredir_write manages to call back into itself
via spice; this patch causes the recursion to fail (0 bytes) the write;
this seems to avoid the deadlock I was previously seeing.
I can't say I fully understand the interaction of usbredir and spice;
but there are a few similar guards in spice and usbredir
to catch other cases especially onces also related to spice_server_char_device_wakeup
This case seems to be triggered by repeated migration+repeated
reconnection of the viewer; but my debugging suggests the migration
finished before this hits.
The backtrace of the hang looks like:
reds_handle_ticket
reds_handle_other_links
reds_channel_do_link
red_channel_connect
spicevmc_connect
usbredir_create_parser
usbredirparser_do_write
usbredir_write
qemu_chr_fe_write
qemu_chr_write
qemu_chr_write_buffer
spice_chr_write
spice_server_char_device_wakeup
red_char_device_wakeup
red_char_device_write_to_device
vmc_write
usbredirparser_do_write
usbredir_write
qemu_chr_fe_write
qemu_chr_write
qemu_chr_write_buffer
qemu_mutex_lock_impl
and we fail as we lang through qemu_chr_write_buffer's lock
twice.
Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1752320
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20191218113012.13331-1-dgilbert@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
394642a8d3