On Mon, 29 Apr 2019 16:01:29 +1000
Alexey Kardashevskiy <aik@xxxxxxxxx> wrote:
> On 20/04/2019 01:34, Greg Kurz wrote:
> > Since 902bdc57451c, get_pci_dev() calls pci_get_domain_bus_and_slot(). This
> > has the effect of incrementing the reference count of the PCI device, as
> > explained in drivers/pci/search.c:
> >
> > * Given a PCI domain, bus, and slot/function number, the desired PCI
> > * device is located in the list of PCI devices. If the device is
> > * found, its reference count is increased and this function returns a
> > * pointer to its data structure. The caller must decrement the
> > * reference count by calling pci_dev_put(). If no device is found,
> > * %NULL is returned.
> >
> > Nothing was done to call pci_dev_put() and the reference count of GPU and
> > NPU PCI devices rockets up.
> >
> > A natural way to fix this would be to teach the callers about the change,
> > so that they call pci_dev_put() when done with the pointer. This turns
> > out to be quite intrusive, as it affects many paths in npu-dma.c,
> > pci-ioda.c and vfio_pci_nvlink2.c.
>
>
> afaict this referencing is only done to protect the current traverser
> and what you've done is actually a natural way (and the generic
> pci_get_dev_by_id() does exactly the same), although this looks a bit weird.
>
Not exactly the same: pci_get_dev_by_id() always increment the refcount
of the returned PCI device. The refcount is only decremented when this
device is passed to pci_get_dev_by_id() to continue searching.
That means that the users of the PCI device pointer returned by
pci_get_dev_by_id() or its exported variants pci_get_subsys(),
pci_get_device() and pci_get_class() do handle the refcount. They
all pass the pointer to pci_dev_put() or continue the search,
which calls pci_dev_put() internally.
Direct and indirect callers of get_pci_dev() don't care for the
refcount at all unless I'm missing something.
>
> > Also, the issue appeared in 4.16 and
> > some affected code got moved around since then: it would be problematic
> > to backport the fix to stable releases.
> >
> > All that code never cared for reference counting anyway. Call pci_dev_put()
> > from get_pci_dev() to revert to the previous behavior.
> >> Fixes: 902bdc57451c ("powerpc/powernv/idoa: Remove unnecessary pcidev
> from pci_dn")
> > Cc: stable@xxxxxxxxxxxxxxx # v4.16
> > Signed-off-by: Greg Kurz <groug@xxxxxxxx>
> > ---
> > arch/powerpc/platforms/powernv/npu-dma.c | 15 ++++++++++++++-
> > 1 file changed, 14 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/powerpc/platforms/powernv/npu-dma.c b/arch/powerpc/platforms/powernv/npu-dma.c
> > index e713ade30087..d8f3647e8fb2 100644
> > --- a/arch/powerpc/platforms/powernv/npu-dma.c
> > +++ b/arch/powerpc/platforms/powernv/npu-dma.c
> > @@ -31,9 +31,22 @@ static DEFINE_SPINLOCK(npu_context_lock);
> > static struct pci_dev *get_pci_dev(struct device_node *dn)
> > {
> > struct pci_dn *pdn = PCI_DN(dn);
> > + struct pci_dev *pdev;
> >
> > - return pci_get_domain_bus_and_slot(pci_domain_nr(pdn->phb->bus),
> > + pdev = pci_get_domain_bus_and_slot(pci_domain_nr(pdn->phb->bus),
> > pdn->busno, pdn->devfn);
> > +
> > + /*
> > + * pci_get_domain_bus_and_slot() increased the reference count of
> > + * the PCI device, but callers don't need that actually as the PE
> > + * already holds a reference to the device.
>
> Imho this would be just enough.
>
> Anyway,
>
> Reviewed-by: Alexey Kardashevskiy <aik@xxxxxxxxx>
>
Thanks !
I now realize that I forgot to add the --cc option for stable on my stgit
command line :-\.
Cc'ing now.
>
> How did you find it? :)
>
While reading code to find some inspiration for OpenCAPI passthrough. :)
I saw the following in vfio_pci_ibm_npu2_init():
if (!pnv_pci_get_gpu_dev(vdev->pdev))
return -ENODEV;
and simply followed the function calls.
>
> > Since callers aren't
> > + * aware of the reference count change, call pci_dev_put() now to
> > + * avoid leaks.
> > + */
> > + if (pdev)
> > + pci_dev_put(pdev);
> > +
> > + return pdev;
> > }
> >
> > /* Given a NPU device get the associated PCI device. */
> >
>
