tracing: Add a way to soft disable trace events

In order to let triggers enable or disable events, we need a 'soft'
method for doing so. For example, if a function probe is added that
lets a user enable or disable events when a function is called, that
change must be done without taking locks or a mutex, and definitely
it can't sleep. But the full enabling of a tracepoint is expensive.

By adding a 'SOFT_DISABLE' flag, and converting the flags to be updated
without the protection of a mutex (using set/clear_bit()), this soft
disable flag can be used to allow critical sections to enable or disable
events from being traced (after the event has been placed into "SOFT_MODE").

Some caveats though: The comm recorder (to map pids with a comm) can not
be soft disabled (yet). If you disable an event with with a "soft"
disable and wait a while before reading the trace, the comm cache may be
replaced and you'll get a bunch of <...> for comms in the trace.

Reading the "enable" file for an event that is disabled will now give
you "0*" where the '*' denotes that the tracepoint is still active but
the event itself is "disabled".

[ fixed _BIT used in & operation : thanks to Dan Carpenter and smatch ]

Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/include/trace/ftrace.h b/include/trace/ftrace.h
index bbf09c2021b9..4bda044e6c77 100644
--- a/include/trace/ftrace.h
+++ b/include/trace/ftrace.h
@@ -413,6 +413,10 @@ static inline notrace int ftrace_get_offsets_##call(			\
  *      int __data_size;
  *      int pc;
  *
+ *      if (test_bit(FTRACE_EVENT_FL_SOFT_DISABLED_BIT,
+ *                   &ftrace_file->flags))
+ *              return;
+ *
  *      local_save_flags(irq_flags);
  *      pc = preempt_count();
  *
@@ -518,6 +522,10 @@ ftrace_raw_event_##call(void *__data, proto)				\
         int __data_size;                                                \
         int pc;                                                         \
                                                                         \
+        if (test_bit(FTRACE_EVENT_FL_SOFT_DISABLED_BIT,                 \
+                     &ftrace_file->flags))                              \
+                return;                                                 \
+                                                                        \
         local_save_flags(irq_flags);                                    \
         pc = preempt_count();                                           \
                                                                         \