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 tried many configurations, like disabling PREEMPT_RT and other stuff which might affect the call stack in this case, but the only two methods which actually avoid the deadlock are: 1. don't call wacom_intuos_schedule_prox_event() / hid_hw_request() from the wacom driver 2. shrink the critical region to not cover hid_input_report() inside hid_ctrl() I am very open to any ideas on how to better fix this, just to be able to use a mainline kernel with my device without out of tree patching :) -- To unsubscribe from this list: send the line "unsubscribe linux-input" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
