Oliver Neukum <oneukum@xxxxxxxx> writes: > On 15.04.24 08:47, Bjørn Mork wrote: > >> urb from service_outstanding_interrupt(). That's why it was added. See >> the explanation Robert wrote when introducing it: >> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/usb/class/cdc-wdm.c?id=c1da59dad0ebd3f9bd238f3fff82b1f7ffda7829 > > Well, the explanation is correct in that we must read > data available. However, if the RESPONDING flag is set > and the URB submitted, we are already doing so. Sounds reasonable. Except that the bug proves we didn't. If you are right that service_outstanding_interrupt can race againts itself (and I don't doubt that), then I guess this could also happen between failure to submit the URB and clearing the flag? >> As for the XMM behaviour: it's been a long time since I tried any of >> those, but AFAIR one the major differences compared to Qualcomm was the >> strict queue handling in the firmware. This caused a number of problems >> where the cdc-wdm driver wanted to skip a message for some reason. So >> I'm not surprised that a bug like this is triggered by one of those >> modems. That's probably the only thing they are good for :-) > > I am not sure where exactly the issue lies here. Suggestions for > debugging? Nothing more than the obvious; Get one of those modems and do some usbmon snooping. Bjørn
