ftrace: Add private data to ftrace_ops

Passing data to the function callback was originally done by adding the
ftrace_ops in another structure, and using the container_of() to get
the field. But this adds a bit more complexity than it is worth, and
adding a simple .private field to ftrace_ops makes things a lot easier.

But be warned, the .private data should not be freed once it is used
unless the ftrace_ops itself has gone through the necessary freeing
routines. A simple synchronize_sched() is not enough as functions
can be traced that are called outside the view of RCU and all its
concoctions.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index f4233b195dab..ef1607ed7044 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -105,11 +105,23 @@ enum {
         FTRACE_OPS_FL_INITIALIZED               = 1 << 8,
 };
 
+/*
+ * Note, ftrace_ops can be referenced outside of RCU protection.
+ * (Although, for perf, the control ops prevent that). If ftrace_ops is
+ * allocated and not part of kernel core data, the unregistering of it will
+ * perform a scheduling on all CPUs to make sure that there are no more users.
+ * Depending on the load of the system that may take a bit of time.
+ *
+ * Any private data added must also take care not to be freed and if private
+ * data is added to a ftrace_ops that is in core code, the user of the
+ * ftrace_ops must perform a schedule_on_each_cpu() before freeing it.
+ */
 struct ftrace_ops {
         ftrace_func_t                   func;
         struct ftrace_ops               *next;
         unsigned long                   flags;
         int __percpu                    *disabled;
+        void                            *private;
 #ifdef CONFIG_DYNAMIC_FTRACE
         struct ftrace_hash              *notrace_hash;
         struct ftrace_hash              *filter_hash;