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

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Message-ID: <49057136.9090107@intel.com>
Date:	Mon, 27 Oct 2008 15:43:50 +0800
From:	"Zhao, Yu" <yu.zhao@...el.com>
To:	Matthew Wilcox <matthew@....cx>
CC:	Yu Zhao <yu.zhao@...scape.net>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"greg@...ah.com" <greg@...ah.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"stable@...nel.org" <stable@...nel.org>,
	Rakib Mullick <rakib.mullick@...il.com>
Subject: Re: [PATCH] pci: Fixing drivers/pci/search.c compilation warning.

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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