Re: [PATCH v2] hid: usbhid: hid-core: fix recursive deadlock

Jiri Kosina
2015-12-01T16:36:21Z

On Fri, 20 Nov 2015, Ioan-Adrian Ratiu wrote:

> The critical section protected by usbhid->lock in hid_ctrl() is too
> big and because of this it causes a recursive deadlock. "Too big" means
> the case statement and the call to hid_input_report() do not need to be
> protected by the spinlock (no URB operations are done inside them).
>
> The deadlock happens because in certain rare cases drivers try to grab
> the lock while handling the ctrl irq which grabs the lock before them
> as described above. For example newer wacom tablets like 056a:033c try
> to reschedule proximity reads from wacom_intuos_schedule_prox_event()
> calling hid_hw_request() -> usbhid_request() -> usbhid_submit_report()
> which tries to grab the usbhid lock already held by hid_ctrl().
>
> There are two ways to get out of this deadlock:
> 1. Make the drivers work "around" the ctrl critical region, in the
> wacom case for ex. by delaying the scheduling of the proximity read
> request itself to a workqueue.
> 2. Shrink the critical region so the usbhid lock protects only the
> instructions which modify usbhid state, calling hid_input_report()
> with the spinlock unlocked, allowing the device driver to grab the
> lock first, finish and then grab the lock afterwards in hid_ctrl().
>
> This patch implements the 2nd solution.
>
> Signed-off-by: Ioan-Adrian Ratiu <adi@adirat.com>

Applied to for-4.5/core branch. Thanks,

--
Jiri Kosina
SUSE Labs