Windows: Fix deadlock in backend when submitting transfers.

Without this change the Windows backend needed to call usbi_fd_notification()
from within the backend's submit_transfer. This can cause deadlock when
attempting to lock the event lock if another thread was processing events on
the just-submitted transfer.

The deadlock comes about as the thread calling libusb_submit_transfer acquires
the transfer mutex before trying to acquire the event lock; this is the other
order of lock acquisition from an event thread handling activity on the just
submitted transfer. This could lead to one of two deadlocks:

1) If the transfer completes while usbi_fd_notification() is waiting for
the event lock and the callback attempts to resubmit the transfer.

2) If the transfer timeout is hit while usbi_fd_notification() is waiting
for the event lock then the attempt to cancel the transfer will deadlock.

This patch fixes both of these deadlocks by having libusb_submit_transfer()
only call usbi_fd_notification() after having released the transfer mutex.

1 files changed, 3 insertions, 0 deletions

diff --git a/libusb/libusbi.h b/libusb/libusbi.h
index c3d2158..0aa6bf9 100644
--- a/libusb/libusbi.h
+++ b/libusb/libusbi.h
@@ -347,6 +347,9 @@ enum usbi_transfer_flags {
 
 	/* Operation on the transfer failed because the device disappeared */
 	USBI_TRANSFER_DEVICE_DISAPPEARED = 1 << 3,
+
+	/* Set by backend submit_transfer() if the fds in use have been updated */
+	USBI_TRANSFER_UPDATED_FDS = 1 << 4,
 };
 
 #define USBI_TRANSFER_TO_LIBUSB_TRANSFER(transfer) \