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SubjectRe: [PATCH] powerpc/powernv/npu: Fix reference leak
Date
Greg Kurz <groug@kaod.org> writes:
> Michael,
>
> Any comments on this patch ? Should I repost with a shorter comment
> as suggested by Alexey ?

No the longer comment seems fine to me.

I'm not a big fan of the patch, it's basically a hack :)

But for a backportable fix I guess it is OK.

I would be happier though if we eventually fix up the code to do the
refcounting properly.

cheers

> On Mon, 29 Apr 2019 12:36:59 +0200
> Greg Kurz <groug@kaod.org> wrote:
>> On Mon, 29 Apr 2019 16:01:29 +1000
>> Alexey Kardashevskiy <aik@ozlabs.ru> 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@vger.kernel.org # v4.16
>> > > Signed-off-by: Greg Kurz <groug@kaod.org>
>> > > ---
>> > > 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@ozlabs.ru>
>> >
>>
>> 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. */
>> > >
>> >
>>

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