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Date:   Mon, 13 May 2019 13:56:06 +0200
From:   Greg Kurz <groug@kaod.org>
To:     Michael Ellerman <mpe@ellerman.id.au>
Cc:     linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org, Alistair Popple <alistair@popple.id.au>,
        Alexey Kardashevskiy <aik@ozlabs.ru>
Subject: Re: [PATCH] powerpc/powernv/npu: Fix reference leak
Message-ID: <20190513135606.7d9a0902@bahia.lan>
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        <20190429123659.00c0622b@bahia.lan>
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Michael,

Any comments on this patch ? Should I repost with a shorter comment
as suggested by Alexey ?

Cheers,

--
Greg

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. */
> > >     
> >   
>