linux-kernel - Re: [PATCH] PCI: pciehp: Avoid returning prematurely from sysfs requests

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Date:   Fri, 9 Aug 2019 10:28:15 -0700
From:   sathyanarayanan kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Lukas Wunner <lukas@...ner.de>, Bjorn Helgaas <helgaas@...nel.org>
Cc:     linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Xiongfeng Wang <wangxiongfeng2@...wei.com>
Subject: Re: [PATCH] PCI: pciehp: Avoid returning prematurely from sysfs
 requests

Hi,

On 8/9/19 3:28 AM, Lukas Wunner wrote:
> A sysfs request to enable or disable a PCIe hotplug slot should not
> return before it has been carried out.  That is sought to be achieved
> by waiting until the controller's "pending_events" have been cleared.
>
> However the IRQ thread pciehp_ist() clears the "pending_events" before
> it acts on them.  If pciehp_sysfs_enable_slot() / _disable_slot() happen
> to check the "pending_events" after they have been cleared but while
> pciehp_ist() is still running, the functions may return prematurely
> with an incorrect return value.
Can this be fixed by changing the sequence of clearing the 
pending_events in pciehp_ist() ?
>
> Fix by introducing an "ist_running" flag which must be false before a
> sysfs request is allowed to return.
>
> Fixes: 32a8cef274fe ("PCI: pciehp: Enable/disable exclusively from IRQ thread")
> Link: https://lore.kernel.org/linux-pci/1562226638-54134-1-git-send-email-wangxiongfeng2@huawei.com
> Reported-and-tested-by: Xiongfeng Wang <wangxiongfeng2@...wei.com>
> Signed-off-by: Lukas Wunner <lukas@...ner.de>
> Cc: stable@...r.kernel.org # v4.19+
> ---
>   drivers/pci/hotplug/pciehp.h      | 2 ++
>   drivers/pci/hotplug/pciehp_ctrl.c | 6 ++++--
>   drivers/pci/hotplug/pciehp_hpc.c  | 2 ++
>   3 files changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/pci/hotplug/pciehp.h b/drivers/pci/hotplug/pciehp.h
> index 8c51a04b8083..e316bde45c7b 100644
> --- a/drivers/pci/hotplug/pciehp.h
> +++ b/drivers/pci/hotplug/pciehp.h
> @@ -72,6 +72,7 @@ extern int pciehp_poll_time;
>    * @reset_lock: prevents access to the Data Link Layer Link Active bit in the
>    *	Link Status register and to the Presence Detect State bit in the Slot
>    *	Status register during a slot reset which may cause them to flap
> + * @ist_running: flag to keep user request waiting while IRQ thread is running
>    * @request_result: result of last user request submitted to the IRQ thread
>    * @requester: wait queue to wake up on completion of user request,
>    *	used for synchronous slot enable/disable request via sysfs
> @@ -101,6 +102,7 @@ struct controller {
>   
>   	struct hotplug_slot hotplug_slot;	/* hotplug core interface */
>   	struct rw_semaphore reset_lock;
> +	unsigned int ist_running;
>   	int request_result;
>   	wait_queue_head_t requester;
>   };
> diff --git a/drivers/pci/hotplug/pciehp_ctrl.c b/drivers/pci/hotplug/pciehp_ctrl.c
> index 631ced0ab28a..1ce9ce335291 100644
> --- a/drivers/pci/hotplug/pciehp_ctrl.c
> +++ b/drivers/pci/hotplug/pciehp_ctrl.c
> @@ -368,7 +368,8 @@ int pciehp_sysfs_enable_slot(struct hotplug_slot *hotplug_slot)
>   		ctrl->request_result = -ENODEV;
>   		pciehp_request(ctrl, PCI_EXP_SLTSTA_PDC);
>   		wait_event(ctrl->requester,
> -			   !atomic_read(&ctrl->pending_events));
> +			   !atomic_read(&ctrl->pending_events) &&
> +			   !ctrl->ist_running);
>   		return ctrl->request_result;
>   	case POWERON_STATE:
>   		ctrl_info(ctrl, "Slot(%s): Already in powering on state\n",
> @@ -401,7 +402,8 @@ int pciehp_sysfs_disable_slot(struct hotplug_slot *hotplug_slot)
>   		mutex_unlock(&ctrl->state_lock);
>   		pciehp_request(ctrl, DISABLE_SLOT);
>   		wait_event(ctrl->requester,
> -			   !atomic_read(&ctrl->pending_events));
> +			   !atomic_read(&ctrl->pending_events) &&
> +			   !ctrl->ist_running);
>   		return ctrl->request_result;
>   	case POWEROFF_STATE:
>   		ctrl_info(ctrl, "Slot(%s): Already in powering off state\n",
> diff --git a/drivers/pci/hotplug/pciehp_hpc.c b/drivers/pci/hotplug/pciehp_hpc.c
> index bd990e3371e3..9e2d7688e8cc 100644
> --- a/drivers/pci/hotplug/pciehp_hpc.c
> +++ b/drivers/pci/hotplug/pciehp_hpc.c
> @@ -608,6 +608,7 @@ static irqreturn_t pciehp_ist(int irq, void *dev_id)
>   	irqreturn_t ret;
>   	u32 events;
>   
> +	ctrl->ist_running = true;
>   	pci_config_pm_runtime_get(pdev);
>   
>   	/* rerun pciehp_isr() if the port was inaccessible on interrupt */
> @@ -654,6 +655,7 @@ static irqreturn_t pciehp_ist(int irq, void *dev_id)
>   	up_read(&ctrl->reset_lock);
>   
>   	pci_config_pm_runtime_put(pdev);
> +	ctrl->ist_running = false;
>   	wake_up(&ctrl->requester);
>   	return IRQ_HANDLED;
>   }

-- 
Sathyanarayanan Kuppuswamy
Linux kernel developer