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Date: Fri, 11 Jul 2014 16:48:01 -0400
From: Boris Ostrovsky <boris.ostrovsky@...cle.com>
To: konrad@...nel.org, xen-devel@...ts.xenproject.org,
david.vrabel@...rix.com, linux-kernel@...r.kernel.org
CC: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH v4 2/5] xen/pciback: Don't deadlock when unbinding.
On 07/11/2014 04:08 PM, konrad@...nel.org wrote:
> From: Konrad Rzeszutek Wilk <konrad.wilk@...cle.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@...cle.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;
--
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