[lttng-dev] Debian specific userspace RCU configure override (ARMv7 Beagle Bone Black)

[Thibault, Daniel]

>   Recall that building lttng-ust on a Beagle Bone Black (Angstrom GNU/Linux v2012.12 (Core edition) on kernel 3.8.13), 
>the make fails at the CCLD of libringbuffer_la-ring_buffer_frontend.lo with an unexpected (and so far unexplained) 
>group of "Error: selected processor does not support ARM mode 'dmb'" (25 times).
>
> [...]
>   At this point I can only try lttng-tools in its "kernel tracing only" mode.  [...]
>
>   I run into trouble quickly, however.  The CC of common/hashtable/rculfhash.lo runs into "Error: selected processor 
>does not support ARM mode 'dmb'" (8 times).  The make aborts at this point.

   Progress on this front.  I did one thing: I edited userspace-rcu/urcu/arch/arm.h to comment out the #ifdef CONFIG_RCU_ARM_HAVE_DMB defines so that userspace-rcu builds "dmb-less".  Once rebuilt and re-installed, I can then move on to try lttng-tools again, and this time it passes the rculfhash.lo step.  It dies much later for a completely unrelated reason which I expect to fix shortly.  At this point I also expect a rebuild of lttng-ust would succeed.

   So it looks like the problem is that the makefiles of lttng-tools and lttng-ust somehow impose too strong an architecture constraint on their linker (i.e. dmb-less generic ARM instead of ARMv7 with dmb).

   I hope this helps.

Daniel U. Thibault
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