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

From: Ioan-Adrian Ratiu
Date: Wed Nov 18 2015 - 14:26:10 EST

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.

Signed-off-by: Ioan-Adrian Ratiu <adi@xxxxxxxxxx>
drivers/hid/usbhid/hid-core.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/hid/usbhid/hid-core.c b/drivers/hid/usbhid/hid-core.c
index 36712e9..5dd426f 100644
--- a/drivers/hid/usbhid/hid-core.c
+++ b/drivers/hid/usbhid/hid-core.c
@@ -477,8 +477,6 @@ static void hid_ctrl(struct urb *urb)
struct usbhid_device *usbhid = hid->driver_data;
int unplug = 0, status = urb->status;

- spin_lock(&usbhid->lock);
switch (status) {
case 0: /* success */
if (usbhid->ctrl[usbhid->ctrltail].dir == USB_DIR_IN)
@@ -498,6 +496,8 @@ static void hid_ctrl(struct urb *urb)
hid_warn(urb->dev, "ctrl urb status %d received\n", status);

+ spin_lock(&usbhid->lock);
if (unplug) {
usbhid->ctrltail = usbhid->ctrlhead;
} else {

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