Re: [PATCH] uio:uio_pci_generic:Don't clear master bit when the process does not exit

[Suweifeng (Weifeng, EulerOS)]

On 2023/2/14 21:17, Greg KH wrote:
> On Tue, Feb 14, 2023 at 09:21:57PM +0800, Su Weifeng wrote:
> > From: Weifeng Su <suweifeng1@xxxxxxxxxx>
> >
> > The /dev/uioX device is used by multiple processes. The current behavior
> > is to clear the master bit when a process exits. This affects other
> > processes that use the device, resulting in command suspension and
> > timeout. This behavior cannot be sensed by the process itself.
> > The solution is to add the reference counting. The reference count is
> > self-incremented and self-decremented each time when the device open and
> > close. The master bit is cleared only when the last process exited.
> >
> > Signed-off-by: Weifeng Su <suweifeng1@xxxxxxxxxx>
> > ---
> >   drivers/uio/uio_pci_generic.c | 18 +++++++++++++++++-
> >   1 file changed, 17 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/uio/uio_pci_generic.c b/drivers/uio/uio_pci_generic.c
> > index e03f9b532..d36d3e08e 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  dev_refc;
> >   };
> >   
> >   static inline struct uio_pci_generic_dev *
> > @@ -39,10 +40,22 @@ to_uio_pci_generic_dev(struct uio_info *info)
> >   	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);
> > +
> > +	if (gdev)
> > +		refcount_inc(&gdev->dev_refc);
>
> This flat out does not work, sorry.
>
> You should never rely on trying to count open/release calls, just let
> the vfs layer handle that for us as it currently does so.
>
> Think about what happens if you call dup() in userspace on a
> filehandle...
>
> > +	return 0;
> > +}
> > +
> >   static int release(struct uio_info *info, struct inode *inode)
> >   {
> >   	struct uio_pci_generic_dev *gdev = to_uio_pci_generic_dev(info);
> >   
> > +	if (gdev && refcount_dec_not_one(&gdev->dev_refc))
>
> I don't think you actually tested this as it is impossible for gdev to
> ever be NULL.
>
> sorry, but this patch is not correct.
>
> greg k-h

First of all, thank you for taking the time to read this patch, your 
comments are very enlightening, but I do have a strange problem here, I 
test such programs on kernels 5.10 and 6.2.
fd = open("/dev/uio0". O_RDWR);
while (true)
	sleep(1);
This program only opens the uio device. After starting multiple such 
processes, I close one of them. From the added print, it can be seen 
that the "uio_pci_generic.c:release" function is called and the master 
bit is cleared, instead of being called when the last process exits as 
expected. I think the vfs is not protected as it should be. Such a 
problem cannot be handled in the user-mode program. We have to clear the 
master bit when the last process exits. Otherwise, user-mode programs 
(for example, the DPDK process that uses uio_pci_generic) cannot work 
properly.

Best regards,
Weifeng Su