From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753081Ab1HYJUK (ORCPT ); Thu, 25 Aug 2011 05:20:10 -0400 Received: from david.siemens.de ([192.35.17.14]:31550 "EHLO david.siemens.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750898Ab1HYJUI (ORCPT ); Thu, 25 Aug 2011 05:20:08 -0400 Message-ID: <4E5613BA.5070101@siemens.com> Date: Thu, 25 Aug 2011 11:19:54 +0200 From: Jan Kiszka User-Agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv:1.8.1.12) Gecko/20080226 SUSE/2.0.0.12-1.1 Thunderbird/2.0.0.12 Mnenhy/0.7.5.666 MIME-Version: 1.0 To: Brian King CC: Linux Kernel Mailing List , "linux-pci@vger.kernel.org" , Alex Williamson , "Michael S. Tsirkin" , Jesse Barnes , Matthew Wilcox Subject: Re: Broken pci_block_user_cfg_access interface References: <4E54D5D7.8050807@siemens.com> <4E551298.2000302@linux.vnet.ibm.com> In-Reply-To: <4E551298.2000302@linux.vnet.ibm.com> Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2011-08-24 17:02, Brian King wrote: > On 08/24/2011 05:43 AM, Jan Kiszka wrote: >> Hi, >> >> trying to port the generic device interrupt masking pattern of >> uio_pci_generic to KVM's device assignment code, I stumbled over some >> fundamental problem with the current pci_block/unblock_user_cfg_access >> interface: it does not provide any synchronization between blocking >> sides. This allows user space to trigger a kernel BUG, just run two >> >> while true; do echo 1 > /sys/bus/pci/devices//reset; done >> >> loops in parallel and watch the kernel oops. >> >> Instead of some funky open-coded locking mechanism, we would rather need >> a plain mutex across both the user space access (via sysfs) and the >> sections guarded by pci_block/unblock_user_cfg_access so far. But I'm >> not sure which of them already allow sleeping, specifically if the IPR >> driver would be fine with such a change. Can someone in the CC list >> comment on this? > > The ipr driver calls pci_block/unblock_user_cfg_access from interrupt > context, so a mutex won't work. Ugh. What precisely does it have to do with the config space while running inside an IRQ handler (or holding a lock that synchronizes it with such a handler)? > When the pci_block/unblock API was > originally added, it did not have the checking it has today to detect > if it is being called nested. This was added some time later. The For a reason... > API that works best for the ipr driver is to allow for many block calls, > but a single unblock call unblocks access. It seems like what might > work well in the case above is a block count. Each call to pci_block > increments a count. Each pci_unblock decrements the count and only > actually do the unblock if the count drops to zero. It should be reasonably > simple for ipr to use that sort of an API as well. That will just paper over the underlying bug: multiple kernel users (!= sysfs access) fiddle with the config space in an unsynchronized fashion. Think of sysfs-triggered pci_reset_function while your ipr driver does its accesses. So it's pointless to tweak the current pci_block semantics, we rather need to establish a new mechanism that synchronizes *all* users of the config space. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux