On Mon, Apr 11, 2011 at 12:25:07PM -0400, Alan Stern wrote:
> On Mon, 11 Apr 2011, Roedel, Joerg wrote:
>
> > > > +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.
>
> I don't follow you. Sure it is finished; this is the "commit" part of
> the probe.
Nevermind, I thought you were refering to the spin-locked part at the
beginning of the function.
> > 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.
>
> I meant the increment should be done before the "if" statement but
> after the spin_lock_irqsave(). That way nobody else can jump in at the
> wrong time.
In the real commit case the
amd_chipset = info;
line will overwrite the increment if the probe is done before the
if-statement. So incrementing amd_chipset.probe_count directly only
matters for the case where we detected a race.
> > > > + 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.
>
> Do it however you prefer. But as it stands now, the patch is wrong.
Hmm, I see how it can be done differently, but no real bug.
Joerg
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