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Date:   Thu, 31 Mar 2022 16:29:23 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Yao Hongbo <yaohongbo@linux.alibaba.com>
Cc:     gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org,
        alikernel-developer@linux.alibaba.com
Subject: Re: [PATCH] uio/uio_pci_generic: Introduce refcnt on open/release
Message-ID: <20220331162521-mutt-send-email-mst@kernel.org>
References: <1648692384-109967-1-git-send-email-yaohongbo@linux.alibaba.com>
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On Thu, Mar 31, 2022 at 10:06:24AM +0800, Yao Hongbo wrote:
> If two userspace programs both open the PCI UIO fd, when one
> of the program exits uncleanly, the other will cause IO hang
> due to bus-mastering disabled.

With two programs poking at the same device, how is this ever
supposed to work even while both are alive?

> It's a common usage for spdk/dpdk to use UIO.

Except people really should just use vfio ...

> So, introduce refcnt
> to avoid such problems.
> 
> Fixes: 865a11f("uio/uio_pci_generic: Disable bus-mastering on release")
> Reported-by: Xiu Yang <yangxiu.yx@alibaba-inc.com>
> Signed-off-by: Yao Hongbo <yaohongbo@linux.alibaba.com>
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>  drivers/uio/uio_pci_generic.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/uio/uio_pci_generic.c b/drivers/uio/uio_pci_generic.c
> index e03f9b5..8add2cf 100644
> --- a/drivers/uio/uio_pci_generic.c
> +++ b/drivers/uio/uio_pci_generic.c
> @@ -31,6 +31,7 @@
>  struct uio_pci_generic_dev {
>  	struct uio_info info;
>  	struct pci_dev *pdev;
> +	atomic_t refcnt;
>  };
>  
>  static inline struct uio_pci_generic_dev *
> @@ -39,6 +40,14 @@ struct uio_pci_generic_dev {
>  	return container_of(info, struct uio_pci_generic_dev, info);
>  }
>  
> +static int open(struct uio_info *info, struct inode *inode)
> +{
> +	struct uio_pci_generic_dev *gdev = to_uio_pci_generic_dev(info);
> +
> +	atomic_inc(&gdev->refcnt);
> +	return 0;
> +}
> +
>  static int release(struct uio_info *info, struct inode *inode)
>  {
>  	struct uio_pci_generic_dev *gdev = to_uio_pci_generic_dev(info);
> @@ -51,7 +60,9 @@ static int release(struct uio_info *info, struct inode *inode)
>  	?* Note that there's a non-zero chance doing this will wedge the device
>  	?* at least until reset.
>  	 */
> -	pci_clear_master(gdev->pdev);
> +	if (atomic_dec_and_test(&gdev->refcnt))
> +		pci_clear_master(gdev->pdev);
> +
>  	return 0;
>  }
>  
> @@ -92,8 +103,11 @@ static int probe(struct pci_dev *pdev,
>  
>  	gdev->info.name = "uio_pci_generic";
>  	gdev->info.version = DRIVER_VERSION;
> +	gdev->info.open = open;
>  	gdev->info.release = release;
>  	gdev->pdev = pdev;
> +	atomic_set(&gdev->refcnt, 0);
> +
>  	if (pdev->irq && (pdev->irq != IRQ_NOTCONNECTED)) {
>  		gdev->info.irq = pdev->irq;
>  		gdev->info.irq_flags = IRQF_SHARED;
> -- 
> 1.8.3.1