On Thu, Apr 07, 2011 at 05:01:01AM -0400, Xu, Andiry wrote:
> > -----Original Message-----
> > From: Roedel, Joerg [mailto:[email protected]]
> > Sent: Thursday, April 07, 2011 3:51 PM
> > To: Alan Stern
> > Cc: Greg Kroah-Hartman; Sarah Sharp; Xu, Andiry; [email protected];
> > [email protected]; [email protected]
> > Subject: Re: [PATCH] USB host: Fix lockdep warning in AMD PLL quirk
> >
> > On Wed, Apr 06, 2011 at 11:16:07AM -0400, Alan Stern wrote:
> > > On Wed, 6 Apr 2011, Joerg Roedel wrote:
> >
> > > The counter really should be a bool: Has the chipset already been
> > > probed or not? After all, nobody cares how many times this routine was
> > > called.
> > >
> > > > + spin_unlock_irqrestore(&amd_lock, flags);
> > >
> > > This code now contains a bug: You incremented the probe_count _before_
> > > doing the probe. If another thread calls this routine right now, it
> > > will get an incorrect result.
> > >
> > > Fixing this up should be fairly easy.
> >
> > Hmm, we can get rid of the amd_lock completly if every thread uses the
> > following call-order:
> >
> > usb_amd_find_chipset_info();
> > usb_amd_quirk_pll_enable();
> > usb_amd_quirk_pll_disable();
> > usb_amd_dev_put();
> >
>
> The correct order is:
>
> usb_amd_find_chipset_info();
> usb_amd_quirk_pll_disable();
> usb_amd_quirk_pll_enable();
> usb_amd_dev_put();
>
> The pair of pll disable and enable may be called for multiple times.
So we could access the data structure without any locks if we want using
atomic_t for the probe_count and isoc_reqs members. But as I've seen
meanwhile the lock still needs to protect the access to the hardware in
the usb_amd_quirk_pll() function.
So its probably not worth the work, what do you think?
Regards,
Joerg
--
AMD Operating System Research Center
Advanced Micro Devices GmbH Einsteinring 24 85609 Dornach
General Managers: Alberto Bozzo, Andrew Bowd
Registration: Dornach, Landkr. Muenchen; Registerger. Muenchen, HRB Nr. 43632
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