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=ham 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 1F268C47404 for ; Wed, 9 Oct 2019 12:49:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E748721848 for ; Wed, 9 Oct 2019 12:49:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1570625382; bh=BS4jso9hypNio5lXJwUJxoGbh/2wvN6PY7GhM+KUNGo=; h=Date:From:To:Cc:Subject:In-Reply-To:List-ID:From; b=TUss11RmXmzly0MNgEA8wlrJHoaR4uNeEhS7RLLunHLzutNhhtrnpO87a5Ir2p40c yWVJxMD8ef10xvgAO0ipynWESi14OjEnf7pw1EDJEyRHVMao/HxsjuJUXMOyzJohHe 78qgXRtE1s9NmKKcpsyyXglysc0VyWeOXje4uaO8= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730606AbfJIMtl (ORCPT ); Wed, 9 Oct 2019 08:49:41 -0400 Received: from mail.kernel.org ([198.145.29.99]:48454 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729784AbfJIMtl (ORCPT ); Wed, 9 Oct 2019 08:49:41 -0400 Received: from localhost (173-25-83-245.client.mchsi.com [173.25.83.245]) (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 9C2C920B7C; Wed, 9 Oct 2019 12:49:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1570625380; bh=BS4jso9hypNio5lXJwUJxoGbh/2wvN6PY7GhM+KUNGo=; h=Date:From:To:Cc:Subject:In-Reply-To:From; b=jH2fwKYGgUAT3yvVwlPaj40RpZcpMd28Pv1P0XTRASzDbfUec6elVvevVNgQPnl5L SUolQjQLJky2IL2PxOIjStpZNm3Mnb717g61/tSzlO+i2tTIQjv1Iuy6WMQv+h9rJA n0oJqlQTsS8MU9UPyPVj6ija2LN0Ad0y9AwzM7es= Date: Wed, 9 Oct 2019 07:49:38 -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: <20191009124938.GA67585@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-pm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-pm@vger.kernel.org On Wed, Oct 09, 2019 at 12:54:37AM +0200, Rafael J. Wysocki wrote: > On Tue, Oct 8, 2019 at 11:16 PM Bjorn Helgaas wrote: > > > > 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. > > In principle, pcie_aspm_enabled() need not be called via sysfs. > > In the particular NVMe use case, it is called from the driver's own PM > callback, so it would be safe without the locking AFAICS. Right, pcie_aspm_enabled() is only used by drivers (actually only by the nvme driver so far). And aspm_attr_show_common() is only used via new sysfs code being added by Heiner. > I guess it is safe to drop the locking from there, but then it would > be good to mention in the kerneldoc that calling it is only safe under > the assumption that the link_state object cannot go away while it is > running. I'll post a patch to that effect. Thanks! Bjorn