Hello,
while playing around with multiple fifo contexts I got fifo hangs when opening glxgears nr.3.
The reason is that in drm/linux_core/nouveau_fifo.c l.342 engine->graph.create_context() returns 
-ENOMEM (I dont know why but I think this problems also appears with other errors).
But the real problem is that then nouveau_fifo alloc() returns without reenabling fifo execution, 
so all fifos hang. 
With this patch glxgears nr.3 only prints scary messages and falls back to software rendering.

Greetings
Stephan Schmid