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

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Message-ID: <alpine.LNX.2.00.1511182137020.20111@pobox.suse.cz>
Date:	Wed, 18 Nov 2015 21:37:42 +0100 (CET)
From:	Jiri Kosina <jikos@...nel.org>
To:	Ioan-Adrian Ratiu <adi@...rat.com>
cc:	pinglinux@...il.com, linux-usb@...r.kernel.org,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hid: usbhid: hid-core: fix recursive deadlock

On Wed, 18 Nov 2015, Ioan-Adrian Ratiu wrote:

> 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.

I think the proper fix actually is to spin_lock_irqsave() in hid_ctrl(), 
isn't it?

-- 
Jiri Kosina
SUSE Labs

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