Severe reproducible nouveau breakage in 2.6.36 (and maybe .35)

[Andrew Lutomirski]

On Wed, Nov 10, 2010 at 2:28 PM, Andrew Lutomirski <andy at luto.us> wrote:
> Hi all-
>
> Somewhere between 2.6.34-fedora-whatever and 2.6.36, Nouveau became
> extremely broken on my hardware.  It appears to be triggered by a bug
> in my monitor (HP LP2475w), which causes the monitor to disappear from
> DVI when it goes to sleep.  Every time the console blanks (in X or
> otherwise AFAICT) the system crashes oddly but unrecoverably.  This is
> 100% reproducible by Ctrl-Alt-F2 followed by 'echo 1
>>/sys/class/graphics/fb0/blank' *from SSH* and waiting a few seconds
> for the monitor to go to sleep, but it also happens if I just walk
> away from the computer long enough for it to blank itself.  This is
> present on F14's kernel and on 2.6.36 from kernel.org.  This may or
> may not be related to the unreproducible crashes that I used to get
> rarely on 2.6.34.

>
> The best hint I have is from this patch (sorry for whitespace damage):
>

>
> which spews "nv50 got hpd irq" once the display blanks.

I tracked it down.  The interrupt code in 2.6.36 is totally broken ---
it acknowledges the interrupt *in the bottom half*.  This might work
by accident if the bottom half gets queued on a different CPU, but
something probably changed (concurrency-managed workqueues?) that make
the BH end up on the same cpu.  So the cpu starves the BH and there
goes a cpu.

Then the clocksource watchdog hits and takes the whole system down
when it calls stop_machine, which also gets starved on that cpu.

Patch coming.

--Andy