Patch "HID: usbhid: fix recursive deadlock" has been added to the 4.3-stable tree

This is a note to let you know that I've just added the patch titled

HID: usbhid: fix recursive deadlock

to the 4.3-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
hid-usbhid-fix-recursive-deadlock.patch
and it can be found in the queue-4.3 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Ioan-Adrian Ratiu <[email protected]>
Date: Fri, 20 Nov 2015 22:19:02 +0200
Subject: HID: usbhid: fix recursive deadlock

From: Ioan-Adrian Ratiu <[email protected]>

commit e470127e9606b1fa151c4184243e61296d1e0c0f upstream.

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.

---
 drivers/hid/usbhid/hid-core.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -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 {

Patches currently in stable-queue which might be from [email protected] are

queue-4.3/hid-usbhid-fix-recursive-deadlock.patch