From 30203c01fa1bb2a7b92575683f85695a2d420b38 Mon Sep 17 00:00:00 2001 From: "Dr. David Alan Gilbert" Date: Wed, 18 Dec 2019 11:30:12 +0000 Subject: [PATCH] usbredir: Prevent recursion in usbredir_write 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 Message-Id: <20191218113012.13331-1-dgilbert@redhat.com> Signed-off-by: Gerd Hoffmann --- hw/usb/redirect.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/hw/usb/redirect.c b/hw/usb/redirect.c index 9764a57987..3cf82589ed 100644 --- a/hw/usb/redirect.c +++ b/hw/usb/redirect.c @@ -109,6 +109,7 @@ struct USBRedirDevice { /* Properties */ CharBackend cs; bool enable_streams; + bool in_write; uint8_t debug; int32_t bootindex; char *filter_str; @@ -286,6 +287,13 @@ 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) { @@ -296,6 +304,7 @@ static int usbredir_write(void *priv, uint8_t *data, int count) r = 0; } } + dev->in_write = false; return r; } -- 2.27.0