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

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Message-ID: <490467CD.3000402@uniscape.net>
Date:	Sun, 26 Oct 2008 20:51:25 +0800
From:	Yu Zhao <yu.zhao@...scape.net>
To:	Matthew Wilcox <matthew@....cx>
CC:	Jesse Barnes <jbarnes@...tuousgeek.org>, linux-pci@...r.kernel.org,
	greg@...ah.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, stable@...nel.org,
	Rakib Mullick <rakib.mullick@...il.com>
Subject: Re: [PATCH] pci: Fixing drivers/pci/search.c compilation warning.

Matthew Wilcox wrote:
> This patch seems to have been overlooked.  It also seems to have had
> some kind of midair collision with a patch from Greg that ignored the
> real bug I found.
> 
> Here's an updated version.  I think it should also be applied to
> -stable.
> 
> ----
> 
> Subject: [PCI] Fix reference counting bug
> 
> pci_get_subsys() will decrement the reference count of the device that
> it starts searching from.  Unfortunately, the pci_find_device() interface 
> will already have decremented the reference count of the device earlier,
> so the device will end up losing all reference counts and be freed.
> 
> We can fix this by incrementing the reference count of the device to
> start searching from before calling pci_get_subsys().
> 
> Signed-off-by: Matthew Wilcox <willy@...ux.intel.com>
> 
> diff --git a/drivers/pci/search.c b/drivers/pci/search.c
> index 4edfc47..5af8bd5 100644
> --- a/drivers/pci/search.c
> +++ b/drivers/pci/search.c
> @@ -166,6 +166,7 @@ struct pci_dev *pci_find_device(unsigned int vendor, unsigned int device,
>  {
>  	struct pci_dev *pdev;
>  
> +	pci_dev_get(from);
>  	pdev = pci_get_subsys(vendor, device, PCI_ANY_ID, PCI_ANY_ID, from);
>  	pci_dev_put(pdev);
>  	return pdev;
> @@ -270,12 +271,8 @@ static struct pci_dev *pci_get_dev_by_id(const struct pci_device_id *id,
>  	struct pci_dev *pdev = NULL;
>  
>  	WARN_ON(in_interrupt());
> -	if (from) {
> -		/* FIXME
> -		 * take the cast off, when bus_find_device is made const.
> -		 */
> -		dev_start = (struct device *)&from->dev;
> -	}
> +	if (from)
> +		dev_start = &from->dev;
>  	dev = bus_find_device(&pci_bus_type, dev_start, (void *)id,
>  			      match_pci_dev_by_id);
>  	if (dev)

This reminds me of other problems of PCI search functions.

The 'dev_start' is passed to bus_find_device(), and its 'knode_bus' 
reference count is decreased by klist_iter_init_node() in that function. 
The problem is the reference count may be already decrease to 0 because 
the PCI device 'from' is hot-plugged off (e.g., pci_remove_bus) when the 
search goes. A warning is fired when klist_iter_init_node() detects the 
reference count becomes 0.

Some code uses pci_find_device() in a way that is not safe with the 
hotplug, because a device may be destroyed after bus_find_device() 
returns it and before it's held by pci_dev_get() in the next round. 
Following is an example from a random grep:

for ( ;; )
{
	if ((dev_netjet = pci_find_device(PCI_VENDOR_ID_TIGERJET,
			PCI_DEVICE_ID_TIGERJET_300,  dev_netjet))) {
		ret = njs_pci_probe(dev_netjet, cs);
		...
	}
	...
}

And some others use pci_get_bus_and_slot(), which has similar problem as 
above.


Thanks,
Yu
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