On Wed, 18 Nov 2015 17:58:56 -0600 Josh Cartwright <joshc@xxxxxx> wrote: > On Wed, Nov 18, 2015 at 11:05:44PM +0200, Ioan-Adrian Ratiu wrote: > > On Wed, 18 Nov 2015 21:37:42 +0100 (CET) > > Jiri Kosina <jikos@xxxxxxxxxx> wrote: > > > > > On Wed, 18 Nov 2015, Ioan-Adrian Ratiu wrote: > > > > > > > The critical section protected by usbhid->lock in hid_ctrl() is too > > > > big and in rare cases causes a recursive deadlock because of its call > > > > to hid_input_report(). > > > > > > > > This deadlock reproduces on newer wacom tablets like 056a:033c because > > > > the wacom driver in its irq handler ends up calling hid_hw_request() > > > > from wacom_intuos_schedule_prox_event() in wacom_wac.c. What this means > > > > is that it submits a report to reschedule a proximity read through a > > > > sync ctrl call which grabs the lock in hid_ctrl(struct urb *urb) > > > > before calling hid_input_report(). When the irq kicks in on the same > > > > cpu, it also tries to grab the lock resulting in a recursive deadlock. > > > > > > > > The proper fix is to shrink the critical section in hid_ctrl() to > > > > protect only the instructions which modify usbhid, thus move the lock > > > > after the hid_input_report() call and the deadlock dissapears. > > > > > > I think the proper fix actually is to spin_lock_irqsave() in hid_ctrl(), > > > isn't it? > > > > > > > That was my first attempt, yes, but the deadlock still happens with > > interrupts disabled. It is very weird, I know. > > I think your best course of action is to figure out why this is the > case, instead of continuing with trying to solve the symptoms. Do you > have actual callstacks showing the cases where you hit? That might be > useful to share (your lockdep picture cuts out the callstacks). > > Also, have you tried without the PREEMPT_RT patch in the picture at all? > > Josh Yes, of course I tried it without PREEMPT_RT_FULL :) This happens on vanilla mainline kernels (only after 4.4-rc1 which introduced support for this kind of tablets). I also backported all the wacom patches to 4.1 non-RT and the same deadlock happens. I've sent another email with some lockdep traces and printk's on a running vanilla linux-next, maybe it didn't get through, here are the links again: First part of lockdep report: http://imgur.com/clLsCWe Second part: http://imgur.com/Wa2PzRl Here are some printk's of mine while reproducing + debugging the issue: http://imgur.com/SETOHT7 I'll continue to research this more in depth, but progress is slow because I don't have much time, I'm doing this in my spare time because it's my girlfriend's tablet. -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
