tracing: add trace_event_read_lock()

I found that there is nothing to protect event_hash in
ftrace_find_event(). Rcu protects the event hashlist
but not the event itself while we use it after its extraction
through ftrace_find_event().

This lack of a proper locking in this spot opens a race
window between any event dereferencing and module removal.

Eg:

--Task A--

print_trace_line(trace) {
  event = find_ftrace_event(trace)

--Task B--

trace_module_remove_events(mod) {
  list_trace_events_module(ev, mod) {
    unregister_ftrace_event(ev->event) {
      hlist_del(ev->event->node)
        list_del(....)
    }
  }
}
|--> module removed, the event has been dropped

--Task A--

  event->print(trace); // Dereferencing freed memory

If the event retrieved belongs to a module and this module
is concurrently removed, we may end up dereferencing a data
from a freed module.

RCU could solve this, but it would add latency to the kernel and
forbid tracers output callbacks to call any sleepable code.
So this fix converts 'trace_event_mutex' to a read/write semaphore,
and adds trace_event_read_lock() to protect ftrace_find_event().

[ Impact: fix possible freed memory dereference in ftrace ]

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <4A114806.7090302@cn.fujitsu.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>