From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754971Ab2K0WMR (ORCPT ); Tue, 27 Nov 2012 17:12:17 -0500 Received: from g1t0027.austin.hp.com ([15.216.28.34]:5109 "EHLO g1t0027.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753284Ab2K0WMO (ORCPT ); Tue, 27 Nov 2012 17:12:14 -0500 Message-ID: <1354053827.26955.196.camel@misato.fc.hp.com> Subject: Re: [RFC PATCH v3 3/3] acpi_memhotplug: Allow eject to proceed on rebind scenario From: Toshi Kani To: Vasilis Liaskovitis Cc: Wen Congyang , Wen Congyang , linux-acpi@vger.kernel.org, isimatu.yasuaki@jp.fujitsu.com, rjw@sisk.pl, lenb@kernel.org, gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Date: Tue, 27 Nov 2012 15:03:47 -0700 In-Reply-To: <20121127183245.GA4674@dhcp-192-168-178-175.profitbricks.localdomain> References: <1353693037-21704-1-git-send-email-vasilis.liaskovitis@profitbricks.com> <1353693037-21704-4-git-send-email-vasilis.liaskovitis@profitbricks.com> <50B0F3DF.4000802@gmail.com> <20121126083634.GA4574@dhcp-192-168-178-175.profitbricks.localdomain> <50B3323E.7020907@cn.fujitsu.com> <1353975541.26955.182.camel@misato.fc.hp.com> <20121127183245.GA4674@dhcp-192-168-178-175.profitbricks.localdomain> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.4.4 (3.4.4-2.fc17) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2012-11-27 at 19:32 +0100, Vasilis Liaskovitis wrote: > On Mon, Nov 26, 2012 at 05:19:01PM -0700, Toshi Kani wrote: > > > >> Consider the following sequence of operations for a hotplugged memory > > > >> device: > > > >> > > > >> 1. echo "PNP0C80:XX" > /sys/bus/acpi/drivers/acpi_memhotplug/unbind > > > >> 2. echo 1 >/sys/bus/pci/devices/PNP0C80:XX/eject > > > >> > > > >> If we don't offline/remove the memory, we have no chance to do it in > > > >> step 2. After > > > >> step2, the memory is used by the kernel, but we have powered off it. It > > > >> is very > > > >> dangerous. > > > > > > > > How does power-off happen after unbind? acpi_eject_store checks for existing > > > > driver before taking any action: > > > > > > > > #ifndef FORCE_EJECT > > > > if (acpi_device->driver == NULL) { > > > > ret = -ENODEV; > > > > goto err; > > > > } > > > > #endif > > > > > > > > FORCE_EJECT is not defined afaict, so the function returns without scheduling > > > > acpi_bus_hot_remove_device. Is there another code path that calls power-off? > > > > > > Consider the following case: > > > > > > We hotremove the memory device by SCI and unbind it from the driver at the same time: > > > > > > CPUa CPUb > > > acpi_memory_device_notify() > > > unbind it from the driver > > > acpi_bus_hot_remove_device() > > > > Can we make acpi_bus_remove() to fail if a given acpi_device is not > > bound with a driver? If so, can we make the unbind operation to perform > > unbind only? > > acpi_bus_remove_device could check if the driver is present, and return -ENODEV > if it's not present (dev->driver == NULL). > > But there can still be a race between an eject and an unbind operation happening > simultaneously. This seems like a general problem to me i.e. not specific to an > acpi memory device. How do we ensure an eject does not race with a driver unbind > for other acpi devices? > > Is there a per-device lock in acpi-core or device-core that can prevent this from > happening? Driver core does a device_lock(dev) on all operations, but this is > probably not grabbed on SCI-initiated acpi ejects. Since driver_unbind() calls device_lock(dev->parent) before calling device_release_driver(), I am wondering if we can call device_lock(dev->dev->parent) at the beginning of acpi_bus_remove() (i.e. before calling pre_remove) and fails if dev->driver is NULL. The parent lock is otherwise released after device_release_driver() is done. Thanks, -Toshi