linux-kernel - Re: [PATCH v11 2/6] xenbus/backend: Protect xenbus callback with lock

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Date:   Tue, 17 Dec 2019 17:13:42 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     SeongJae Park <sjpark@...zon.com>, axboe@...nel.dk,
        konrad.wilk@...cle.com, roger.pau@...rix.com
Cc:     SeongJae Park <sjpark@...zon.de>, pdurrant@...zon.com,
        sj38.park@...il.com, xen-devel@...ts.xenproject.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 2/6] xenbus/backend: Protect xenbus callback with lock

On 17.12.19 17:07, SeongJae Park wrote:
> From: SeongJae Park <sjpark@...zon.de>
> 
> 'reclaim_memory' callback can race with a driver code as this callback
> will be called from any memory pressure detected context.  To deal with
> the case, this commit adds a spinlock in the 'xenbus_device'.  Whenever
> 'reclaim_memory' callback is called, the lock of the device which passed
> to the callback as its argument is locked.  Thus, drivers registering
> their 'reclaim_memory' callback should protect the data that might race
> with the callback with the lock by themselves.
> 
> Signed-off-by: SeongJae Park <sjpark@...zon.de>
> ---
>   drivers/xen/xenbus/xenbus_probe.c         |  1 +
>   drivers/xen/xenbus/xenbus_probe_backend.c | 10 ++++++++--
>   include/xen/xenbus.h                      |  2 ++
>   3 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
> index 5b471889d723..b86393f172e6 100644
> --- a/drivers/xen/xenbus/xenbus_probe.c
> +++ b/drivers/xen/xenbus/xenbus_probe.c
> @@ -472,6 +472,7 @@ int xenbus_probe_node(struct xen_bus_type *bus,
>   		goto fail;
>   
>   	dev_set_name(&xendev->dev, "%s", devname);
> +	spin_lock_init(&xendev->reclaim_lock);
>   
>   	/* Register with generic device framework. */
>   	err = device_register(&xendev->dev);
> diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
> index 7e78ebef7c54..516aa64b9967 100644
> --- a/drivers/xen/xenbus/xenbus_probe_backend.c
> +++ b/drivers/xen/xenbus/xenbus_probe_backend.c
> @@ -251,12 +251,18 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
>   static int backend_reclaim_memory(struct device *dev, void *data)
>   {
>   	const struct xenbus_driver *drv;
> +	struct xenbus_device *xdev;
> +	unsigned long flags;
>   
>   	if (!dev->driver)
>   		return 0;
>   	drv = to_xenbus_driver(dev->driver);
> -	if (drv && drv->reclaim_memory)
> -		drv->reclaim_memory(to_xenbus_device(dev));
> +	if (drv && drv->reclaim_memory) {
> +		xdev = to_xenbus_device(dev);
> +		spin_trylock_irqsave(&xdev->reclaim_lock, flags);

You need spin_lock_irqsave() here. Or maybe spin_lock() would be fine,
too? I can't see a reason why you'd want to disable irqs here.

> +		drv->reclaim_memory(xdev);
> +		spin_unlock_irqrestore(&xdev->reclaim_lock, flags);
> +	}
>   	return 0;
>   }
>   
> diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
> index c861cfb6f720..d9468313061d 100644
> --- a/include/xen/xenbus.h
> +++ b/include/xen/xenbus.h
> @@ -76,6 +76,8 @@ struct xenbus_device {
>   	enum xenbus_state state;
>   	struct completion down;
>   	struct work_struct work;
> +	/* 'reclaim_memory' callback is called while this lock is acquired */
> +	spinlock_t reclaim_lock;
>   };
>   
>   static inline struct xenbus_device *to_xenbus_device(struct device *dev)
> 


Juergen