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

Hi Jiri,

On Tue, Dec 1, 2015 at 8:36 AM, Jiri Kosina [off-list ref] wrote:

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

quoted

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 <redacted>

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.

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