linux-kernel - Re: [PATCH v2] uio/uio_pci_generic: Introduce refcnt on open/release

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Date:   Wed, 13 Apr 2022 04:51:16 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Yao Hongbo <yaohongbo@...ux.alibaba.com>,
        alikernel-developer@...ux.alibaba.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] uio/uio_pci_generic: Introduce refcnt on open/release

On Wed, Apr 13, 2022 at 09:33:17AM +0200, Greg KH wrote:
> On Wed, Apr 13, 2022 at 03:01:42PM +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.
> > 
> > It's a common usage for spdk/dpdk to use UIO. So, introduce refcnt
> > to avoid such problems.
> 
> Why do you have multiple userspace programs opening the same device?
> Shouldn't they coordinate?

Or to restate, I think the question is, why not open the device
once and pass the FD around?


> > 
> > Fixes: 865a11f987ab("uio/uio_pci_generic: Disable bus-mastering on release")


space missing before ( here .

> > Reported-by: Xiu Yang <yangxiu.yx@...baba-inc.com>
> > Signed-off-by: Yao Hongbo <yaohongbo@...ux.alibaba.com>
> > ---
> > Changes for v2:
> > 	Use refcount_t instead of atomic_t to catch overflow/underflows.
> > ---
> >  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..1a5e1fd 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;
> > +	refcount_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);
> > +
> > +	refcount_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 (refcount_dec_and_test(&gdev->refcnt))
> > +		pci_clear_master(gdev->pdev);
> 
> The goal here is to flush things when userspace closes the device, as
> the comment says.  So don't you want that to happen for when userspace
> closes the file handle no matter who opened it?
> 
> As this is a functional change, how is userspace going to "know" this
> functionality is now changed or not?
> 
> And if userspace really wants to open this multiple times, then properly
> switch the code to only create the device-specific structures when open
> is called.  Otherwise you are sharing structures here that are not
> intended to be shared, shouldn't you have your own private one?
> 
> this feels odd.
> 
> thanks,
> 
> greg k-h

Sigh. Maybe it was a mistake to do 865a11f987ab to begin with.
Anyone doing DMA with UIO is already on very thin ice.
But oh well.

-- 
MST