On Wed, Dec 01, 2021 at 09:04:27AM -0800, Shang Shi wrote: > This is Shang Shi from Google. As we are examining the Andorid adb > daemon implementation in AOSP, we found that there's a concern on > functionfs that an io_submit syscall on IN/OUT endpoint after endpoint > becomes disabled could end up blocking [1]. AOSP kills a thread to > avoid it blocking on io_submit, which is not an elegant way to exist. > > Is it true that io_submit syscall could end up blocking after endpoint > is disabled? Would opening endpoint file with O_NONBLOCK resolve the > issue? Looking at ffs_epfile_io() it seems that it can block if the endpoint is disabled, and indeed there may be a race there between the disabled notification and the next submit arriving. There was a recent proposed patch [1] which looks like it inadvertently addresses this race, but there are other issues with that patch so it's not a fix for this. >From the code, indeed O_NONBLOCK does avoid any risk of blocking, but it means you can't queue up I/O requests before the gadget is activated, although maybe that's not a problem in practice. [1] https://lore.kernel.org/linux-usb/20211201100205.25448-1-quic_wcheng@xxxxxxxxxxx/ > From the ffs-aio-example "aio_simple.c", it seems that as long as we > read and process control events before any read/write (with > io_submit), we are not worried about any race condition between > endpoint becoming disabled and io_submit. Is this true? I don't think this holds, there will always be a windows where the UDC can change state between reading the ep0 events and attempting I/O on the other endpoint files. Regards, John
