On Wed, 18 May 2016, Roderick Colenbrander wrote: > Many devices use userspace bluetooth stacks like BlueZ or Bluedroid in combination > with uhid. If any of these stacks is used with a HID device for which the driver > performs a HID request as part .probe (or technically another HID operation), > this results in a deadlock situation. The deadlock results in a 5 second timeout > for I/O operations in HID drivers, so isn't fatal, but none of the I/O operations > have a chance of succeeding. > > The root cause for the problem is that uhid only allows for one request to be > processed at a time per uhid instance and locks out other operations. This means > that if a user space is creating a new HID device through 'UHID_CREATE', which > ultimately triggers '.probe' through the HID layer. Then any HID request e.g. a > read for calibration data would trigger a HID operation on uhid again, but it > won't go out to userspace, because it is still stuck in UHID_CREATE. > In addition bluetooth stacks are typically single threaded, so they wouldn't be > able to handle any requests while waiting on uhid. > > Lucikly the UHID spec is somewhat flexible and allows for fixing the issue, > without breaking user space. The idea which the patch implements as discussed > with David Herrmann is to decouple adding of a hid device (which triggers .probe) > from UHID_CREATE. The work will kick off roughly once UHID_CREATE completed (or > else will wait a tiny bit of time in .probe for a lock). A HID driver has to call > HID to call 'hid_hw_start()' as part of .probe once it is ready for I/O, which > triggers UHID_START to user space. Any HID operations should function now within > .probe and won't deadlock because userspace is stuck on UHID_CREATE. > > We verified this patch on Bluedroid with Android 6.0 and on desktop Linux with > BlueZ stacks. Prior to the patch they had the deadlock issue. > > Signed-off-by: Roderick Colenbrander <roderick.colenbrander@xxxxxxxx> Thanks for the fix. I've applied it to hid.git#for-4.8/uhid-offload-hid-device-add -- Jiri Kosina SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html