From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752257AbdHPNn6 (ORCPT ); Wed, 16 Aug 2017 09:43:58 -0400 Received: from mail.kernel.org ([198.145.29.99]:38026 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751649AbdHPNn5 (ORCPT ); Wed, 16 Aug 2017 09:43:57 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 68CAC22B5F Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=helgaas@kernel.org Date: Wed, 16 Aug 2017 08:43:54 -0500 From: Bjorn Helgaas To: Srinath Mannam Cc: bhelgaas@google.com, linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org, bcm-kernel-feedback-list@broadcom.com Subject: Re: [RFC PATCH v3] pci: Concurrency issue during pci enable bridge Message-ID: <20170816134354.GV32525@bhelgaas-glaptop.roam.corp.google.com> References: <1501858648-22228-1-git-send-email-srinath.mannam@broadcom.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1501858648-22228-1-git-send-email-srinath.mannam@broadcom.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Aug 04, 2017 at 08:27:28PM +0530, Srinath Mannam wrote: > Concurrency issue is observed during pci enable bridge called > for multiple pci devices initialization in SMP system. > > Setup details: > - SMP system with 8 ARMv8 cores running Linux kernel(4.11). > - Two EPs are connected to PCIe RC through bridge as shown > in the below figure. > > [RC] > | > [BRIDGE] > | > ----------- > | | > [EP] [EP] > > Issue description: > After PCIe enumeration completed EP driver probe function called > for both the devices from two CPUS simultaneously. > From EP probe function, pci_enable_device_mem called for both the EPs. > This function called pci_enable_bridge enable for all the bridges > recursively in the path of EP to RC. > > Inside pci_enable_bridge function, at two places concurrency issue is > observed. > > Place 1: > CPU 0: > 1. Done Atomic increment dev->enable_cnt > in pci_enable_device_flags > 2. Inside pci_enable_resources > 3. Completed pci_read_config_word(dev, PCI_COMMAND, &cmd) > 4. Ready to set PCI_COMMAND_MEMORY (0x2) in > pci_write_config_word(dev, PCI_COMMAND, cmd) > CPU 1: > 1. Check pci_is_enabled in function pci_enable_bridge > and it is true > 2. Check (!dev->is_busmaster) also true > 3. Gone into pci_set_master > 4. Completed pci_read_config_word(dev, PCI_COMMAND, &old_cmd) > 5. Ready to set PCI_COMMAND_MASTER (0x4) in > pci_write_config_word(dev, PCI_COMMAND, cmd) > > By the time of last point for both the CPUs are read value 0 and > ready to write 2 and 4. > After last point final value in PCI_COMMAND register is 4 instead of 6. > > Place 2: > CPU 0: > 1. Done Atomic increment dev->enable_cnt in > pci_enable_device_flags > > Signed-off-by: Srinath Mannam > --- > drivers/pci/pci.c | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c > index af0cc34..12721df 100644 > --- a/drivers/pci/pci.c > +++ b/drivers/pci/pci.c > @@ -52,6 +52,7 @@ static void pci_pme_list_scan(struct work_struct *work); > static LIST_HEAD(pci_pme_list); > static DEFINE_MUTEX(pci_pme_list_mutex); > static DECLARE_DELAYED_WORK(pci_pme_work, pci_pme_list_scan); > +static DEFINE_MUTEX(pci_bridge_mutex); > > struct pci_pme_device { > struct list_head list; > @@ -1348,10 +1349,11 @@ static void pci_enable_bridge(struct pci_dev *dev) > if (bridge) > pci_enable_bridge(bridge); > > + mutex_lock(&pci_bridge_mutex); > if (pci_is_enabled(dev)) { > if (!dev->is_busmaster) > pci_set_master(dev); > - return; > + goto end; > } > > retval = pci_enable_device(dev); > @@ -1359,6 +1361,8 @@ static void pci_enable_bridge(struct pci_dev *dev) > dev_err(&dev->dev, "Error enabling bridge (%d), continuing\n", > retval); > pci_set_master(dev); > +end: > + mutex_unlock(&pci_bridge_mutex); I think this will deadlock because we're holding pci_bridge_mutex while we call pci_enable_device(), which may recursively call pci_enable_bridge(), which would try to acquire pci_bridge_mutex again. My original suggestion of a mutex in the host bridge would have the same problem. We talked about using device_lock() earlier. You found some problems with that, and I'd like to understand them better. You said: > But the pci_enable_bridge is called in the context of the driver > probe function, we will have nexted lock problem. The driver core does hold device_lock() while calling the driver probe function, in this path: device_initial_probe __device_attach device_lock(dev) # <-- lock __device_attach_driver ... pci_device_probe ... ->probe # driver probe function device_unlock(dev) # <-- unlock I didn't see your patch using device_lock(), but what I had in mind was something like the patch below, where pci_enable_bridge() acquires the device_lock() of the bridge. For the sake of argument, assume a hierarchy: bridge A -> bridge B -> endpoint C Here's what I think will happen: device_lock(C) # driver core ... ->probe(C) # driver probe function pci_enable_device_flags(C) pci_enable_bridge(B) # enable C's upstream bridge device_lock(B) pci_enable_bridge(A) # enable B's upstream bridge device_lock(A) # A has no upstream bridge pci_enable_device(A) do_pci_enable_device(A) # update A PCI_COMMAND pci_set_master(A) # update A PCI_COMMAND device_unlock(A) pci_enable_device(B) # update B PCI_COMMAND pci_set_master(B) # update B PCI_COMMAND device_unlock(B) do_pci_enable_device(C) # update C PCI_COMMAND device_unlock(C) I don't see a nested lock problem here. What am I missing? Bjorn diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c index e8e40dea2842..38154ba628a9 100644 --- a/drivers/pci/pci.c +++ b/drivers/pci/pci.c @@ -1344,6 +1344,7 @@ static void pci_enable_bridge(struct pci_dev *dev) struct pci_dev *bridge; int retval; + device_lock(&dev->dev); bridge = pci_upstream_bridge(dev); if (bridge) pci_enable_bridge(bridge); @@ -1351,7 +1352,7 @@ static void pci_enable_bridge(struct pci_dev *dev) if (pci_is_enabled(dev)) { if (!dev->is_busmaster) pci_set_master(dev); - return; + goto out; } retval = pci_enable_device(dev); @@ -1359,6 +1360,9 @@ static void pci_enable_bridge(struct pci_dev *dev) dev_err(&dev->dev, "Error enabling bridge (%d), continuing\n", retval); pci_set_master(dev); + +out: + device_unlock(&dev->dev); } static int pci_enable_device_flags(struct pci_dev *dev, unsigned long flags)