Re: [PATCH] powerpc/powernv/npu: Fix reference leak

[Michael Ellerman]

Greg Kurz <groug@xxxxxxxx> 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@xxxxxxxx> wrote:
>> 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. */
>> > >     
>> >   
>>