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SubjectRe: [linux-usb-devel] Re: serious 2.6 bug in USB subsystem?
OK, finally a bit of progress.  If you remember back in October 2003 I
reported:

> One-line summary: plug-in your USB keyboard, see your machine die.

> So, I have this non-name USB keyboard (with built-in 2-port USB
> hub) which reliably crashes 2.6.0-test{8,9} on both x86 and ia64.
> In retrospect, it's clear to me that the same keyboard also
> occasionally crashes 2.4 kernels, but there the problem appears
> more seldom. Perhaps once in 10 reboots and once the machine is
> booted and the keyboard is running, it keeps on working. The
> keyboard in question is a BTC 5141H.

After this, I spent a (small) amount of time looking over the HID code
etc to see what could be causing it. I could find nothing wrong so I
gave up, connected another USB keyboard, and basically ignored the
problem. In retrospect, that was Good Thinking, because I was
apparently looking at the wrong code: the problem _does_ appear to be
coming from the USB HCD, not from the HIDeous code.

Specifically, after upgrading to 2.6.4-rc2, _all_ of the ia64 machines
I tested would crash as soon as they had _any_ USB keyboard plugged
in. That is, the problem no longer was limited to the BTC keyboard,
which is special because it has a built-in hub. This was encouraging.

Turns out it's this patch that was causing the crashes:

http://linux.bkbits.net:8080/linux-2.5/cset@1.1619.1.17

That was strange, because even to my USB-untrained eye the patch
looked obviously correct. However, I think the root cause of the
problem really has to do with a race-condition between the controller
and the driver. In particular, if I apply the patch below, my USB
keyboards (including the BTC keyboard) work just fine!

===== drivers/usb/host/ohci-q.c 1.48 vs edited =====
--- 1.48/drivers/usb/host/ohci-q.c Tue Mar 2 05:52:46 2004
+++ edited/drivers/usb/host/ohci-q.c Fri Mar 5 17:25:55 2004
@@ -438,7 +451,7 @@
* behave. frame_no wraps every 2^16 msec, and changes right before
* SF is triggered.
*/
- ed->tick = OHCI_FRAME_NO(ohci->hcca) + 1;
+ ed->tick = OHCI_FRAME_NO(ohci->hcca) + 2;

/* rm_list is just singly linked, for simplicity */
ed->ed_next = ohci->ed_rm_list;
However, I think the root-cause of the problem may be this optimization
in ohci_irq():

/* we can eliminate a (slow) readl() if _only_ WDH caused this irq */
Indeed, if I apply this patch instead:

===== drivers/usb/host/ohci-hcd.c 1.56 vs edited =====
--- 1.56/drivers/usb/host/ohci-hcd.c Tue Mar 2 05:52:40 2004
+++ edited/drivers/usb/host/ohci-hcd.c Fri Mar 5 17:45:09 2004
@@ -584,7 +584,7 @@
int ints;

/* we can eliminate a (slow) readl() if _only_ WDH caused this irq */
- if ((ohci->hcca->done_head != 0)
+ if (0 && (ohci->hcca->done_head != 0)
&& ! (le32_to_cpup (&ohci->hcca->done_head) & 0x01)) {
ints = OHCI_INTR_WDH;

there are no crashes either.

So my theory is that I was seeing this sequence of events:

- HCD signals WDH interrupt & sends DMA to update the frame number in
the host-controller communication area (HCCA)

- host gets interrupt, but skips readl() and hence reads a stale
frame number N instead of the up-to-date value (N+1)

- HCD cancels a transfer descriptor (TD), moves it to the "remove list"
and calculates the frame number at which it can be remove from
the host-controller's list as N+1

- SOF interrupt arrives (probably was pending already?)

- interrupt handler does a readl() and now sees the updated
frame-number N+1

- HCD sees that the cancelled TD's time stamp N+1 is <= the current
current time stamp (N+1) and goes ahead and removes it from the
host-list, while the controller is still looking at the entry being
removed

- HCD ends up dereferencing a bad pointer and ends up reading from
address 0xf0000000, which on our ia64 machines is a read-only area,
which then results in a machine-check abort
Does this sound plausible?

What beats me is why UHCI would have the same issue. I know even less
about UHCI than I do about OHCI but perhaps there is a similar
problem.

--david
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