perf: Fix event->ctx locking

There have been a few reported issues wrt. the lack of locking around
changing event->ctx. This patch tries to address those.

It avoids the whole rwsem thing; and while it appears to work, please
give it some thought in review.

What I did fail at is sensible runtime checks on the use of
event->ctx, the RCU use makes it very hard.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20150123125834.209535886@infradead.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>
(cherry picked from commit f63a8daa5812afef4f06c962351687e1ff9ccb2b)

Bug: 30955111
Bug: 31095224

Signed-off-by: Joao Dias <joaodias@google.com>
Change-Id: I8dfc0aae8d1206c177454e0093dacd82b6129c55
Git-repo: https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git


Git-commit: f63a8daa5812afef4f06c962351687e1ff9ccb2b
[rsiddoji@codeaurora.org: resloved some trival confilits]
Signed-off-by: Ravi Kumar Siddojigari <rsiddoji@codeaurora.org>
Signed-off-by: Pradosh Das <prados@codeaurora.org>