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@xxxxxxxxxxxxxxx>
> > Signed-off-by: Yao Hongbo <yaohongbo@xxxxxxxxxxxxxxxxx>
> > ---
> > 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
