[Bug 221103] xhci_hcd: System lockup under CPU load during usbfs polling of USB devices on AMD platforms

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=221103

--- Comment #33 from Andrew Skalski (voltara@gmail.com) ---
(In reply to Michał Pecio from comment #32)
> Your finding is curious. Does it mean it would still work if you add
> msleep(10) immediately before the failing line and not earlier?

Yes it works there as well, except you need to use mdelay instead of msleep. 
For that reason, and because xhci_set_cmd_ring_deq is used elsewhere, I moved
the msleep just outside the atomic section in xhci_resume for the proof of
concept.

The reason I tried adding the delay was because I noticed enabling debug
logging made the problem go away.  This made me suspect the lockup was timing
related.

Thanks for the link to the other bug reports.  I'll have a read through them.


On a separate topic, I did some further investigation into my crashes to tie up
some loose ends about the trigger:

1) I'm pretty confident that it's just the two "1022:14c9" controllers that are
causing my crashes.  I have been running an unpatched kernel with
'/sys/bus/pci/devices/<pci-id>/power/control == on' just for those two, and it
has been perfectly stable.

2) The crashes that resulted from plugging in USB devices were somewhat of a
red herring.  A daemon on my system (colord) was reacting to the plug events by
scanning everything in /dev/bus/usb.  So in all cases the underlying trigger
was actually the same: resuming the "1022:14c9" controllers by accessing their
device nodes in /dev/bus/usb.

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