From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FAKE_REPLY_C,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5381FECE58C for ; Tue, 8 Oct 2019 21:17:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2A08221835 for ; Tue, 8 Oct 2019 21:17:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1570569421; bh=Uqjzv4/P9OatGQAy44pX7Dr8dKQk8lXYGW74iZDcG4Y=; h=Date:From:To:Cc:Subject:In-Reply-To:List-ID:From; b=OD9kJVgPrpNV91dnV1L28lfsoUj1Yq7ebEUxERSW/tHgzqSqy4i/8yEXGU3wEpXgz gHiFHesexJ0cEL6WUk6HLNsdra0TI1NrezKlYVtN92VGqt9WZ1UXQOPczgVLZbqD4+ hB+e6Y8+MI5LOaxU1cVWb75jFHMB7wPL13TrYKkY= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731133AbfJHVQ7 (ORCPT ); Tue, 8 Oct 2019 17:16:59 -0400 Received: from mail.kernel.org ([198.145.29.99]:54894 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730912AbfJHVQ7 (ORCPT ); Tue, 8 Oct 2019 17:16:59 -0400 Received: from localhost (unknown [69.71.4.100]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id B0D0121721; Tue, 8 Oct 2019 21:16:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1570569418; bh=Uqjzv4/P9OatGQAy44pX7Dr8dKQk8lXYGW74iZDcG4Y=; h=Date:From:To:Cc:Subject:In-Reply-To:From; b=lWsX2rDSQspNs1nahOSrFjhf8LzrmfUK4rnKcRSmKWoYPeMd7QazDZl+mE5hMlzN+ lOMGd6B0PHzFWgjeZbtyG+Ns0nazdRWbh+Cwy6NXB0BdsQ3l3O+vzZfQLBlP/ee2VQ MoEDHX97hkbN2uWl8X1poEgNuuLN8mLLSqedIk/w= Date: Tue, 8 Oct 2019 16:16:56 -0500 From: Bjorn Helgaas To: "Rafael J. Wysocki" Cc: "Rafael J. Wysocki" , linux-nvme , Keith Busch , Mario Limonciello , Kai-Heng Feng , Keith Busch , Christoph Hellwig , Sagi Grimberg , Linux PM , Linux Kernel Mailing List , Rajat Jain , Linux PCI , Heiner Kallweit Subject: Re: [PATCH v3 1/2] PCI: PCIe: ASPM: Introduce pcie_aspm_enabled() Message-ID: <20191008211656.GA163302@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 08, 2019 at 11:27:51AM +0200, Rafael J. Wysocki wrote: > On Tue, Oct 8, 2019 at 12:34 AM Bjorn Helgaas wrote: > > On Thu, Aug 08, 2019 at 11:55:07PM +0200, Rafael J. Wysocki wrote: > > > From: Rafael J. Wysocki > > > > > > Add a function checking whether or not PCIe ASPM has been enabled for > > > a given device. > > > > > > It will be used by the NVMe driver to decide how to handle the > > > device during system suspend. > > > > > > Signed-off-by: Rafael J. Wysocki > > > --- > > > > > > v2 -> v3: > > > * Make the new function return bool. > > > * Change its name back to pcie_aspm_enabled(). > > > * Fix kerneldoc comment formatting. > > > > > > -> v2: > > > * Move the PCI/PCIe ASPM changes to a separate patch. > > > * Add the _mask suffix to the new function name. > > > * Add EXPORT_SYMBOL_GPL() to the new function. > > > * Avoid adding an unnecessary blank line. > > > > > > --- > > > drivers/pci/pcie/aspm.c | 20 ++++++++++++++++++++ > > > include/linux/pci.h | 3 +++ > > > 2 files changed, 23 insertions(+) > > > > > > Index: linux-pm/drivers/pci/pcie/aspm.c > > > =================================================================== > > > --- linux-pm.orig/drivers/pci/pcie/aspm.c > > > +++ linux-pm/drivers/pci/pcie/aspm.c > > > @@ -1170,6 +1170,26 @@ static int pcie_aspm_get_policy(char *bu > > > module_param_call(policy, pcie_aspm_set_policy, pcie_aspm_get_policy, > > > NULL, 0644); > > > > > > +/** > > > + * pcie_aspm_enabled - Check if PCIe ASPM has been enabled for a device. > > > + * @pci_device: Target device. > > > + */ > > > +bool pcie_aspm_enabled(struct pci_dev *pci_device) > > > +{ > > > + struct pci_dev *bridge = pci_upstream_bridge(pci_device); > > > + bool ret; > > > + > > > + if (!bridge) > > > + return false; > > > + > > > + mutex_lock(&aspm_lock); > > > + ret = bridge->link_state ? !!bridge->link_state->aspm_enabled : false; > > > + mutex_unlock(&aspm_lock); > > > > Why do we need to acquire aspm_lock here? We aren't modifying > > anything, and I don't think we're preventing a race. If this races > > with another thread that changes aspm_enabled, we'll return either the > > old state or the new one, and I think that's still the case even if we > > don't acquire aspm_lock. > > Well, if we can guarantee that pci_remove_bus_device() will never be > called in parallel with this helper, then I agree, but can we > guarantee that? Hmm, yeah, I guess that's the question. It's not a race with another thread changing aspm_enabled; the potential race is with another thread removing the last child of "bridge", which will free the link_state and set bridge->link_state = NULL. I think it should be safe to call device-related PCI interfaces if you're holding a reference to the device, e.g., from a driver bound to the device or a sysfs accessor. Since we call pcie_aspm_enabled(dev) from a driver bound to "dev", another thread should not be able to remove "dev" while we're using it. I know that's a little hand-wavey, but if it weren't true, I think we'd have a lot more locking sprinkled everywhere in the PCI core than we do. This has implications for Heiner's ASPM sysfs patches because we're currently doing this in sysfs accessors: static ssize_t aspm_attr_show_common(struct device *dev, ...) { ... link = pcie_aspm_get_link(pdev); mutex_lock(&aspm_lock); enabled = link->aspm_enabled & state; mutex_unlock(&aspm_lock); ... } I assume sysfs must be holding a reference that guarantees "dev" is valid througout this code, and therefore we should not need to hold aspm_lock. Bjorn