From: "Dr. David Alan Gilbert" <dgilbert@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 <dgilbert@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