[ubuntu-x] Heisenbug in xserver-xorg-input-synaptics

Chase Douglas chase.douglas at canonical.com
Sat Jan 7 16:48:31 UTC 2012

On 01/06/2012 06:05 PM, Chase Douglas wrote:
> Hi all,
> I tried to track down the bug in the X staging ppa
> (ppa:ubuntu-x-swat/x-staging) that causes trackpads to flip to the edge
> of the screen. This is what I found:
> In the X server when there's a relative motion event it computes an
> acceleration for it. You can follow the events from QueuePointerEvents()
> to BasicComputeAcceleration(). BasicComputeAcceleration() then calls a
> function pointer, which by default for the X synaptics driver calls back
> into the driver's SynapticsAccelerationProfile() function.
> If you set a breakpoint in BasicComputeAcceleration() with the condition
> of "result > 10000", you'll get a hit. When you continue, your pointer
> will flip to an edge of the screen. Generally, the value of the result
> variable is on the order of +<some value>e+304 or inf when this occurs.
> The next step is to try setting a breakpoint in
> SynapticsAccelerationProfile() with the condition "accelfct > 10000" on
> the return statement.
> I compiled xserver-xorg-input-synaptics with noopt, but I wasn't able to
> trigger the bug. Then, I compiled with the default optimizations, and
> the bug reappeared, but the breakpoint with the condition was never hit.
> I tried removing the condition, and the breakpoint worked as expected.
> I've sent this to both ubuntu-x and ubuntu-devel to ask if anyone has
> any ideas on how to debug this without getting to the level of
> compiler/assembly/linker debugging :).

Nevermind, I should have looked closer at the build log. The signature
of the acceleration callback changed to returning a double instead of a
float. This emitted a warning that I failed to see.

-- Chase

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