From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758417AbcFAO0s (ORCPT ); Wed, 1 Jun 2016 10:26:48 -0400 Received: from mx2.suse.de ([195.135.220.15]:60188 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753240AbcFAO0p (ORCPT ); Wed, 1 Jun 2016 10:26:45 -0400 Date: Wed, 1 Jun 2016 16:26:46 +0200 (CEST) From: Jiri Kosina X-X-Sender: jkosina@pobox.suse.cz To: Roderick Colenbrander cc: dh.herrmann@googlemail.com, benjamin.tissoires@redhat.com, linux-input@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] HID: uhid: Fixes a bug with userspace bluetooth stacks, which causes hangs during certain operations In-Reply-To: <1463602269-19844-1-git-send-email-roderick.colenbrander@sony.com> Message-ID: References: <1463602269-19844-1-git-send-email-roderick.colenbrander@sony.com> User-Agent: Alpine 2.00 (LNX 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 18 May 2016, Roderick Colenbrander wrote: > Many devices use userspace bluetooth stacks like BlueZ or Bluedroid in combination > with uhid. If any of these stacks is used with a HID device for which the driver > performs a HID request as part .probe (or technically another HID operation), > this results in a deadlock situation. The deadlock results in a 5 second timeout > for I/O operations in HID drivers, so isn't fatal, but none of the I/O operations > have a chance of succeeding. > > The root cause for the problem is that uhid only allows for one request to be > processed at a time per uhid instance and locks out other operations. This means > that if a user space is creating a new HID device through 'UHID_CREATE', which > ultimately triggers '.probe' through the HID layer. Then any HID request e.g. a > read for calibration data would trigger a HID operation on uhid again, but it > won't go out to userspace, because it is still stuck in UHID_CREATE. > In addition bluetooth stacks are typically single threaded, so they wouldn't be > able to handle any requests while waiting on uhid. > > Lucikly the UHID spec is somewhat flexible and allows for fixing the issue, > without breaking user space. The idea which the patch implements as discussed > with David Herrmann is to decouple adding of a hid device (which triggers .probe) > from UHID_CREATE. The work will kick off roughly once UHID_CREATE completed (or > else will wait a tiny bit of time in .probe for a lock). A HID driver has to call > HID to call 'hid_hw_start()' as part of .probe once it is ready for I/O, which > triggers UHID_START to user space. Any HID operations should function now within > .probe and won't deadlock because userspace is stuck on UHID_CREATE. > > We verified this patch on Bluedroid with Android 6.0 and on desktop Linux with > BlueZ stacks. Prior to the patch they had the deadlock issue. > > Signed-off-by: Roderick Colenbrander Thanks for the fix. I've applied it to hid.git#for-4.8/uhid-offload-hid-device-add -- Jiri Kosina SUSE Labs