On Wednesday 20 February 2008, Alan Stern wrote:
> On Wed, 20 Feb 2008, David Brownell wrote:
>
> > On Wednesday 20 February 2008, Alan Stern wrote:
> > > > ehci_hcd 0000:00:1d.7: IAA watchdog, lost IAA: status 8029 cmd 10021
> > >
> > > lines in the log brings up some ideas that have been percolating in my
> > > mind for a while. They have to do with the possibility of a race
> > > between the watchdog routine and assertion of IAA.
> >
> > The curious bit IMO being STS_INT (0001), which should also have
> > triggered an IRQ. Suggesting to me that the race might be lower
> > level than that ... at the level of a conflict between the various
> > mechanisms to ack irqs.
>
> Maybe it did trigger an IRQ. Inside the watchdog routine interrupts
> are disabled.
>
> > > In fact, if the timing comes out just wrong then it's possible (on SMP
> > > systems) for an IAA interrupt to arrive when the watchdog
> > > routine has already started running. Then end_unlink_async() might get
> > > called right at the start of a new IAA cycle, or when the reclaim list
> > > is empty.
> >
> > The driver's spinlock should prevent that particular problem from
> > appearing.
>
> I don't think so:
>
> CPU 0 CPU 1
> ----- -----
> Watchdog timer expires
> Timer routine acquires spinlock
> IAA IRQ arrives
> ehci_irq tries to acquire
> spinlock...
There's another condition here, and
another action. The condition is
that ehci->reclaim must first be set;
the action is to clear STS_IAA (and,
given the previous patch, maybe IAAD).
And this "either" is more concisely
written as "call end_unlink_async()"
(point made just for clarity).
> Timer routine either sets
> ehci->reclaim to NULL
> or else starts a new
> IAA cycle
> Timer routine releases spinlock
> and returns
> ehci_irq acquires spinlock
> and sees IAA is set
Can only happen if a new IAA
cycle was started by CPU0, and
the IAA condition triggered
that quickly.
> Call end_unlink_async()!
>
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