Re: [PATCH v4 2/5] xen/pciback: Don't deadlock when unbinding.

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 07/11/2014 04:08 PM, konrad@kernel.org wrote:
> From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>
> As commit 0a9fd0152929db372ff61b0d6c280fdd34ae8bdb
> 'xen/pciback: Document the entry points for 'pcistub_put_pci_dev''
> explained there are four entry points in this function.
> Two of them are when the user fiddles in the SysFS to
> unbind a device which might be in use by a guest or not.
>
> Both 'unbind' states will cause a deadlock as the the PCI lock has
> already been taken, which then pci_device_reset tries to take.
>
> We can simplify this by requiring that all callers of
> pcistub_put_pci_dev MUST hold the device lock. And then
> we can just call the lockless version of pci_device_reset.
>
> To make it even simpler we will modify xen_pcibk_release_pci_dev
> to quality whether it should take a lock or not - as it ends
> up calling xen_pcibk_release_pci_dev and needs to hold the lock.
>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> ---
> [v2: Per David Vrabel's suggestion - use lockless version of reset]
> ---
>   drivers/xen/xen-pciback/passthrough.c |    9 +++++++--
>   drivers/xen/xen-pciback/pci_stub.c    |   11 +++++------
>   drivers/xen/xen-pciback/pciback.h     |    7 ++++---
>   drivers/xen/xen-pciback/vpci.c        |    9 +++++++--
>   drivers/xen/xen-pciback/xenbus.c      |    2 +-
>   5 files changed, 24 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/xen/xen-pciback/passthrough.c b/drivers/xen/xen-pciback/passthrough.c
> index 828dddc..d0c3fb4 100644
> --- a/drivers/xen/xen-pciback/passthrough.c
> +++ b/drivers/xen/xen-pciback/passthrough.c
> @@ -69,7 +69,7 @@ static int __xen_pcibk_add_pci_dev(struct xen_pcibk_device *pdev,
>   }
>   
>   static void __xen_pcibk_release_pci_dev(struct xen_pcibk_device *pdev,
> -					struct pci_dev *dev)
> +					struct pci_dev *dev, bool lock)
>   {
>   	struct passthrough_dev_data *dev_data = pdev->pci_dev_data;
>   	struct pci_dev_entry *dev_entry, *t;
> @@ -87,8 +87,13 @@ static void __xen_pcibk_release_pci_dev(struct xen_pcibk_device *pdev,
>   
>   	mutex_unlock(&dev_data->lock);
>   
> -	if (found_dev)
> +	if (found_dev) {
> +		if (lock)
> +			device_lock(&found_dev->dev);
>   		pcistub_put_pci_dev(found_dev);
> +		if (lock)
> +			device_unlock(&found_dev->dev);
> +	}
>   }
>   
>   static int __xen_pcibk_init_devices(struct xen_pcibk_device *pdev)
> diff --git a/drivers/xen/xen-pciback/pci_stub.c b/drivers/xen/xen-pciback/pci_stub.c
> index d57a173..eebbb41 100644
> --- a/drivers/xen/xen-pciback/pci_stub.c
> +++ b/drivers/xen/xen-pciback/pci_stub.c
> @@ -250,6 +250,8 @@ struct pci_dev *pcistub_get_pci_dev(struct xen_pcibk_device *pdev,
>    *  - 'echo BDF > unbind' with a guest still using it. See pcistub_remove
>    *
>    *  As such we have to be careful.
> + *
> + *  To make this easier, the caller has to hold the device lock.

Should we assert that the lock is being held?

-boris

>    */
>   void pcistub_put_pci_dev(struct pci_dev *dev)
>   {
> @@ -276,11 +278,7 @@ void pcistub_put_pci_dev(struct pci_dev *dev)
>   	/* Cleanup our device
>   	 * (so it's ready for the next domain)
>   	 */
> -
> -	/* This is OK - we are running from workqueue context
> -	 * and want to inhibit the user from fiddling with 'reset'
> -	 */
> -	pci_reset_function(dev);
> +	__pci_reset_function_locked(dev);
>   	pci_restore_state(dev);
>   
>   	/* This disables the device. */
> @@ -567,7 +565,8 @@ static void pcistub_remove(struct pci_dev *dev)
>   			/* N.B. This ends up calling pcistub_put_pci_dev which ends up
>   			 * doing the FLR. */
>   			xen_pcibk_release_pci_dev(found_psdev->pdev,
> -						found_psdev->dev);
> +						found_psdev->dev,
> +						false /* caller holds the lock. */);
>   		}
>   
>   		spin_lock_irqsave(&pcistub_devices_lock, flags);
> diff --git a/drivers/xen/xen-pciback/pciback.h b/drivers/xen/xen-pciback/pciback.h
> index f72af87..58e38d5 100644
> --- a/drivers/xen/xen-pciback/pciback.h
> +++ b/drivers/xen/xen-pciback/pciback.h
> @@ -99,7 +99,8 @@ struct xen_pcibk_backend {
>   		    unsigned int *domain, unsigned int *bus,
>   		    unsigned int *devfn);
>   	int (*publish)(struct xen_pcibk_device *pdev, publish_pci_root_cb cb);
> -	void (*release)(struct xen_pcibk_device *pdev, struct pci_dev *dev);
> +	void (*release)(struct xen_pcibk_device *pdev, struct pci_dev *dev,
> +                        bool lock);
>   	int (*add)(struct xen_pcibk_device *pdev, struct pci_dev *dev,
>   		   int devid, publish_pci_dev_cb publish_cb);
>   	struct pci_dev *(*get)(struct xen_pcibk_device *pdev,
> @@ -122,10 +123,10 @@ static inline int xen_pcibk_add_pci_dev(struct xen_pcibk_device *pdev,
>   }
>   
>   static inline void xen_pcibk_release_pci_dev(struct xen_pcibk_device *pdev,
> -					     struct pci_dev *dev)
> +					     struct pci_dev *dev, bool lock)
>   {
>   	if (xen_pcibk_backend && xen_pcibk_backend->release)
> -		return xen_pcibk_backend->release(pdev, dev);
> +		return xen_pcibk_backend->release(pdev, dev, lock);
>   }
>   
>   static inline struct pci_dev *
> diff --git a/drivers/xen/xen-pciback/vpci.c b/drivers/xen/xen-pciback/vpci.c
> index 51afff9..2fdfcb9 100644
> --- a/drivers/xen/xen-pciback/vpci.c
> +++ b/drivers/xen/xen-pciback/vpci.c
> @@ -145,7 +145,7 @@ out:
>   }
>   
>   static void __xen_pcibk_release_pci_dev(struct xen_pcibk_device *pdev,
> -					struct pci_dev *dev)
> +					struct pci_dev *dev, bool lock)
>   {
>   	int slot;
>   	struct vpci_dev_data *vpci_dev = pdev->pci_dev_data;
> @@ -169,8 +169,13 @@ static void __xen_pcibk_release_pci_dev(struct xen_pcibk_device *pdev,
>   out:
>   	mutex_unlock(&vpci_dev->lock);
>   
> -	if (found_dev)
> +	if (found_dev) {
> +		if (lock)
> +			device_lock(&found_dev->dev);
>   		pcistub_put_pci_dev(found_dev);
> +		if (lock)
> +			device_unlock(&found_dev->dev);
> +	}
>   }
>   
>   static int __xen_pcibk_init_devices(struct xen_pcibk_device *pdev)
> diff --git a/drivers/xen/xen-pciback/xenbus.c b/drivers/xen/xen-pciback/xenbus.c
> index 4a7e6e0..065aae8 100644
> --- a/drivers/xen/xen-pciback/xenbus.c
> +++ b/drivers/xen/xen-pciback/xenbus.c
> @@ -290,7 +290,7 @@ static int xen_pcibk_remove_device(struct xen_pcibk_device *pdev,
>   
>   	/* N.B. This ends up calling pcistub_put_pci_dev which ends up
>   	 * doing the FLR. */
> -	xen_pcibk_release_pci_dev(pdev, dev);
> +	xen_pcibk_release_pci_dev(pdev, dev, true /* use the lock */);
>   
>   out:
>   	return err;