Comment 4 for bug 1778642

This bug seems to be resolved by the following commits from lttng-modules upstream:

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commit 1ee63d8d1b0aaf30564d3879d8e9282d0ddfef1b
Author: Michael Jeanson <email address hidden>
Date: Tue Mar 13 12:14:43 2018 -0400

    Cleanup: comment about CONFIG_HOTPLUG_CPU ifdef

    Signed-off-by: Michael Jeanson <email address hidden>
    Signed-off-by: Mathieu Desnoyers <email address hidden>

commit 894ce5dc825733d1ccab25b1aede9ce0be2a7c4e
Author: Lars Persson <email address hidden>
Date: Sun Mar 11 15:02:43 2018 +0100

    Fix: do not use CONFIG_HOTPLUG_CPU for the new hotplug API

    Kernel configurations without CONFIG_HOTPLUG_CPU throw an unknown
    symbol error when attempting to insert the lttng-trace module:
     lttng_tracer: Unknown symbol lttng_hp_prepare (err 0)
     lttng_tracer: Unknown symbol lttng_hp_online (err 0)

    This was caused by lttng-events and lttng-context-perf-counter not
    agreeing on which preprocessor condition that should guard the use of
    the hotplug API. In fact the API is available also on kernels built
    without CONFIG_HOTPLUG_CPU.

    Signed-off-by: Lars Persson <email address hidden>
    Signed-off-by: Mathieu Desnoyers <email address hidden>
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I will build a test package and attach here a debdiff.