From: "Dr. David Alan Gilbert" <dgilb...@redhat.com> 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 <dgilb...@redhat.com> --- hw/usb/redirect.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/hw/usb/redirect.c b/hw/usb/redirect.c index e0f5ca6f81..97f2c3a7da 100644 --- a/hw/usb/redirect.c +++ b/hw/usb/redirect.c @@ -113,6 +113,7 @@ struct USBRedirDevice { /* Properties */ CharBackend cs; bool enable_streams; + bool in_write; uint8_t debug; int32_t bootindex; char *filter_str; @@ -290,6 +291,13 @@ static int usbredir_write(void *priv, uint8_t *data, int count) return 0; } + /* Recursion check */ + if (dev->in_write) { + DPRINTF("usbredir_write recursion\n"); + return 0; + } + dev->in_write = true; + r = qemu_chr_fe_write(&dev->cs, data, count); if (r < count) { if (!dev->watch) { @@ -300,6 +308,7 @@ static int usbredir_write(void *priv, uint8_t *data, int count) r = 0; } } + dev->in_write = false; return r; } -- 2.23.0
