Re: [PATCH] pci: Fixing drivers/pci/search.c compilation warning.

[Zhao, Yu]

Zhao, Yu wrote:
> Matthew Wilcox wrote:
> > On Mon, Oct 27, 2008 at 03:13:51PM +0800, Zhao, Yu wrote:
> > > Matthew Wilcox wrote:
> > > > On Mon, Oct 27, 2008 at 11:18:43AM +0800, Zhao, Yu wrote:
> > > > > Matthew Wilcox wrote:
> > > > > > Yes, that's why pci_find_device() is deprecated.  But it doesn't also
> > > > > > need to be buggy ;-)
> > > > > How about pci_get_bus_and_slot()? People would meet the problem with it 
> > > > > anyway.
> > > > What problem with it?  It's documented to return the device with an
> > > > increased refcount, and the implementation appears to do exactly that:
> > > The 'dev' returned by pci_get_device() may be destroyed by PCI hotplug. 
> > > I suppose that passing this 'dev' to pci_get_device() in the next loop 
> > > would crash the system, right?
> > Erm, no, the 'dev' cannot be destroyed because the caller has a refcount
> > on it.  The physical device backing it might have gone away.  The dev
>
> Why does the caller have a reference count? I don't see we increase the 
> reference count after the 'dev' is returned by following in 
> pci_get_dev_by_id():
>
>          dev = bus_find_device(&pci_bus_type, dev_start, (void *)id,
>                                match_pci_dev_by_id);
>
> And this 'dev' becomes the 'from' in the next loop, but it may be 
> destroyed before the 'pci_dev_get(from)', isn't it?

I checked the source code, there is no 'pci_dev_get(from)', the 
reference count is increased in bus_find_device().

        while ((dev = next_device(&i)))
                if (match(dev, data) && get_device(dev))

But the essential problem is same: the reference count of 'dev' above 
may be decreased before the 'get_device(dev)', I guess.
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