Re: [PATCH 1/3] msi: add forgotten pci_dev_put(pdev) topopulate_msi_sysfs()

[Veaceslav Falico]

On Wed, Sep 25, 2013 at 05:35:54PM -0600, Bjorn Helgaas wrote:
> On Wed, Sep 25, 2013 at 5:23 PM, Neil Horman <nhorman@xxxxxxxxxxxxx> wrote:
> > On Wed, Sep 25, 2013 at 03:08:05PM -0600, Bjorn Helgaas wrote:
> > > [+cc Neil (he added this code in da8d1c8ba4), Greg]
> > >
> > > On Mon, Sep 16, 2013 at 7:47 PM, Veaceslav Falico <vfalico@xxxxxxxxxx> wrote:
> > > > Before trying to kobject_init_and_add(), we add a reference to pdev via
> > > > pci_dev_get(pdev). However, if it fails to init and/or add the kobject, we
> > > > don't return it back - even on out_unroll.
> > > >
> > > > Fix this by adding pci_dev_put(pdev) before going to unrolling section.
> > > >
> > > > CC: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> > > > CC: linux-pci@xxxxxxxxxxxxxxx
> > > > CC: linux-kernel@xxxxxxxxxxxxxxx
> > > > Signed-off-by: Veaceslav Falico <vfalico@xxxxxxxxxx>
> > > > ---
> > > >  drivers/pci/msi.c | 4 +++-
> > > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
> > > > index d5f90d6..14bf578 100644
> > > > --- a/drivers/pci/msi.c
> > > > +++ b/drivers/pci/msi.c
> > > > @@ -534,8 +534,10 @@ static int populate_msi_sysfs(struct pci_dev *pdev)
> > > >                 pci_dev_get(pdev);
> > > >                 ret = kobject_init_and_add(kobj, &msi_irq_ktype, NULL,
> > > >                                      "%u", entry->irq);
> > > > -               if (ret)
> > > > +               if (ret) {
> > > > +                       pci_dev_put(pdev);
> > > >                         goto out_unroll;
> > > > +               }
> > > >
> > > >                 count++;
> > > >         }
> > >
> > > I don't understand why this code does the pci_dev_get() in the first
> > > place.  The pdev->msi_list of msi_desc structs is private to the
> > > pci_dev, and even without bumping the refcount, there should be no way
> > > for the pci_dev to be freed before the msi_desc.
> > Its been a few years now, but IIRC I did the pci_dev_get/put here to ensure that
> > people didn't try to remove the device prior to freeing all their interrupts
> > (i.e I didn't want a broken driver to go through its remove routine without
> > freeing all its irqs).  That might have been the wrong thing to do, but thats
> > what bubbles to the front of my head when looking at this.
>
> That sounds plausible, but I think I'd rather deal with that by having
> the PCI core remove logic free all the interrupts.  I *think* that's
> already in place, i.e., pci_free_resources() calls
> msi_remove_pci_irq_vectors().  So I propose that we remove the
> pci_dev_get()/put() unless we come up with a more compelling reason
> for it.

As an update - I've found an interesting case why exactly that
kobject_del() might be needed:

in kobject_del() it removes instantly the link to kset - via
kobj_kset_leave(), so that our kset remains without links and, thus, might
be instantly removed.

So, with kobject_del(), our kset (msi_irqs sysfs dir) remains instantly
without any links (i.e. other kobjects) and, when we call kset_unregister()
- it exits instantly (if it's not being hold somewhere elsewhere...).

Without it, kset_unregister() will wait till all the kobjects will be gone.

Now, the fun part starts - if we quickly call pci_disable_msi() and,
afterwards, pci_enable_msi() - we might fail because the msi_irqs kset is
still there, waiting to unregister, and the sysfs dir is still active.

It's used, for example, in tg3_open/tg3_close, which are ndo_open/close,
and are called on enslave/deslave in bonding.

What I get:
[   60.458319] WARNING: CPU: 0 PID: 5552 at fs/sysfs/dir.c:526 sysfs_add_one+0xbb/0xe0()
[   60.458350] sysfs: cannot create duplicate filename '/devices/pci0000:00/0000:00:1c.5/0000:3f:00.0/msi_irqs'

I'll take a deeper look at the issue, though any feedback/advise is
welcome. And I'll hold on with the patchset that removes pci_dev_get/put
and kobject_del.


> > > I also don't understand this nearby code (the same pattern appears in
> > > free_msi_irqs()):
> > >
> > >     out_unroll:
> > >         list_for_each_entry(entry, &pdev->msi_list, list) {
> > >                 if (!count)
> > >                         break;
> > >                 kobject_del(&entry->kobj);
> > >                 kobject_put(&entry->kobj);
> > >                 count--;
> > >         }
> > >
> > > Why do we call kobject_del() here?  The kobject_put() will call
> > > kobject_del() anyway, so it looks redundant.
> > > Documentation/kobject.txt says kobject_del() must be called explicitly
> > > to break a circular reference, but I don't think we have that here.
> >  I think thats exactly why I did it, because of the documentation.  I agree
> > however, it does look redundant.  Harmless, but redundant.
>
> OK, thanks.  I think we should remove it on the grounds that it's not
> needed and removing it will make this code look more similar to other
> callers of kobject_init_and_add(), which means bugs will have fewer
> places to hide.
>
> Thanks, Neil!
>
> Bjorn
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