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

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Message-ID: <49056EF6.3000904@intel.com>
Date:	Mon, 27 Oct 2008 15:34:14 +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.

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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