x86, bts: fix race between per-task and per-cpu branch tracing

x86, bts: fix race between per-task and per-cpu branch tracing

Per-task branch tracing installs a debug store context with the traced
task. This immediately results in the branch trace control bits to be
cleared for the next context switch of that task, if not set before.

Either per-cpu or per-task tracing are allowed at the same time.

An active per-cpu tracing would be disabled even if the per-task tracing
request is rejected and the task debug store context removed.

Check the tracing type (per-cpu or per-task) before installing a task
debug store context.

Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Cc: roland@redhat.com
Cc: eranian@googlemail.com
Cc: oleg@redhat.com
Cc: juan.villacis@intel.com
Cc: ak@linux.jf.intel.com
LKML-Reference: <20090403144552.856000000@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>