linux-kernel - Re: [PATCH v5] USB host: Fix lockdep warning in AMD PLL quirk

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Date:	Mon, 11 Apr 2011 18:16:47 +0200
From:	"Roedel, Joerg" <Joerg.Roedel@....com>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	Borislav Petkov <bp@...en8.de>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	"Xu, Andiry" <Andiry.Xu@....com>,
	USB list <linux-usb@...r.kernel.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5] USB host: Fix lockdep warning in AMD PLL quirk

On Mon, Apr 11, 2011 at 11:49:30AM -0400, Alan Stern wrote:
> On Mon, 11 Apr 2011, Roedel, Joerg wrote:
> 
> > On Mon, Apr 11, 2011 at 02:26:00AM -0400, Borislav Petkov wrote:
> > > You mean 2.6.39-rc2 right? I'm asking because I hit the same warning
> > > with 39-rc2 now too but it didn't appear with .38 - so it has to have
> > > snuck in after the merge window. If so, you don't need the stable tag.
> > 
> > You were right, just tested plain 2.6.38 and it doesn't happen. So I
> > removed the stable tag. Here is the updated patch.
> 
> ...
> 
> > --- a/drivers/usb/host/pci-quirks.c
> > +++ b/drivers/usb/host/pci-quirks.c
> > @@ -84,65 +84,92 @@ int usb_amd_find_chipset_info(void)
> >  {
> 
> ...
> 
> > +commit:
> > +
> > +	spin_lock_irqsave(&amd_lock, flags);
> > +	if (amd_chipset.probe_count > 0) {
> > +		/* race - someone else was faster - drop devices */
> > +
> > +		/* Mark that we where here */
> > +		amd_chipset.probe_count++;
> 
> This line should be moved above the "if" statement, since you always 
> want to increment the count.

No, probe_count can't be incremented here because the probe is not
finished yet. If another thread jumps in after the lock is released and
detects probe_count > 0 while the probe hasn't happened the quirk will
fail. So we need to make sure that amd_chipset.probe_count does not
become > 0 before the probe is finished.

> 
> > +		ret = amd_chipset.probe_result;
> > +
> > +		spin_unlock_irqrestore(&amd_lock, flags);
> > +
> > +		if (info.nb_dev)
> > +			pci_dev_put(info.nb_dev);
> > +		if (info.smbus_dev)
> > +			pci_dev_put(info.smbus_dev);
> > +
> > +	} else {
> > +		/* no race - commit the result */
> > +		info.probe_count++;
> 
> This isn't right, because info.probe_count was initialized to 0.  Maybe 
> amd_chipset.probe_count should be made into a separate variable, not a 
> part of the structure, like amd_lock.

The purpose of the struct is structuring of data. In theory all of its
members could be turned into global variables. The amd_lock is different
because it does not only protect the struct but also access to the
hardware while the quirk is applied/unapplied.


	Joerg

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