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

Ping Cheng
2015-12-02T00:00:34Z

Hi Jiri,

On Tue, Dec 1, 2015 at 8:36 AM, Jiri Kosina <jikos@kernel.org> wrote:
> 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,

Since wacom_intuos_schedule_prox_event() was introduced on March 5,
2015, the call was first used in kernel 3.19. Shouldn't we back-port
this patch to other stable versions?

Thank you.

Ping