Hi,

I'm hitting a NULL pointer dereference when I enable the 
i915_context_free tracepoint (call trace attached). This is caused by 
the fact that the trace tries to access ctx->file_priv, which however 
may have already been deleted (even if the pointer is != NULL). I've 
used that trace extensively back when I've submitted it a few months ago 
without ecountering this issue, but it doesn't seem that there ever was 
a guarantee that the file_priv would be valid at ctx free time, so I'll 
put the blame on my original commit that introduced the trace:

	commit 198c974d7e80a5135fc4a2e69a07ba3e64122f8a
	Author: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
	Date:   Mon Nov 10 13:44:31 2014 +0000

     	drm/i915: Add tracepoints to track a vm during its lifetime

The trace doesn't specifically need the file_priv pointer, it just uses 
it to reach the drm_device pointer to get the device index. To fix the 
issue I've looked for another way to get the drm_device pointer from the 
ctx, but I couldn't find any that is valid for every GEN. Modifying the 
trace to add an extra parameter is out of the question for 2 reasons:
- The only variable available in i915_gem_context_free is the ctx ptr
- Modifying a tracepoint could break ABI

Ideas, anyone?

Thanks,
Daniele