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


> won't be destroyed until its reference count reaches zero, which could
> be any time someone calls pci_dev_put() on it. In the scenario you're
> postulating, it would happen in pci_get_dev_by_id():
>
> if (from)
> pci_dev_put(from);
>
> which is the last time that 'from' is referred to in that callchain.
>


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