Code:  int perf_event_task_disable(void)
 {
 	struct perf_event *event;
 
 	mutex_lock(&current->perf_event_mutex);
	list_for_each_entry(event, &current->perf_event_list, owner_entry)
		perf_event_for_each_child(event, perf_event_disable);
 	mutex_unlock(&current->perf_event_mutex);
 
 	return 0;
}

Commit Message: 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>
CWE ID: CWE-264
 int perf_event_task_disable(void)
 {
	struct perf_event_context *ctx;
 	struct perf_event *event;
 
 	mutex_lock(&current->perf_event_mutex);
	list_for_each_entry(event, &current->perf_event_list, owner_entry) {
		ctx = perf_event_ctx_lock(event);
		perf_event_for_each_child(event, _perf_event_disable);
		perf_event_ctx_unlock(event, ctx);
	}
 	mutex_unlock(&current->perf_event_mutex);
 
 	return 0;
}
