[PATCH v4 8/9] Uprobes/sdt: Document about reference counter

[Ravi Bangoria <ravi.bangoria@linux.ibm.com>]

Reference counter gate the invocation of probe. If present,
by default reference count is 0. Kernel needs to increment
it before tracing the probe and decrement it when done. This
is identical to semaphore in Userspace Statically Defined
Tracepoints (USDT).

Document usage of reference counter.

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
---
 Documentation/trace/uprobetracer.rst | 16 +++++++++++++---
 kernel/trace/trace.c                 |  2 +-
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/Documentation/trace/uprobetracer.rst b/Documentation/trace/uprobetracer.rst
index 98d3f69..0c27c65 100644
--- a/Documentation/trace/uprobetracer.rst
+++ b/Documentation/trace/uprobetracer.rst
@@ -22,15 +22,25 @@ Synopsis of uprobe_tracer
 -------------------------
 ::
 
-  p[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a uprobe
-  r[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a return uprobe (uretprobe)
-  -:[GRP/]EVENT                           : Clear uprobe or uretprobe event
+  p[:[GRP/]EVENT] PATH:OFFSET[(REF_CTR_OFFSET)] [FETCHARGS]
+  r[:[GRP/]EVENT] PATH:OFFSET[(REF_CTR_OFFSET)] [FETCHARGS]
+  -:[GRP/]EVENT
+
+  p : Set a uprobe
+  r : Set a return uprobe (uretprobe)
+  - : Clear uprobe or uretprobe event
 
   GRP           : Group name. If omitted, "uprobes" is the default value.
   EVENT         : Event name. If omitted, the event name is generated based
                   on PATH+OFFSET.
   PATH          : Path to an executable or a library.
   OFFSET        : Offset where the probe is inserted.
+  REF_CTR_OFFSET: Reference counter offset. Optional field. Reference count
+                 gate the invocation of probe. If present, by default
+                 reference count is 0. Kernel needs to increment it before
+                 tracing the probe and decrement it when done. This is
+                 identical to semaphore in Userspace Statically Defined
+                 Tracepoints (USDT).
 
   FETCHARGS     : Arguments. Each probe can have up to 128 args.
    %REG         : Fetch register REG
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index c9336e9..08f3e75 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -4613,7 +4613,7 @@ static int tracing_trace_options_open(struct inode *inode, struct file *file)
   "place (kretprobe): [<module>:]<symbol>[+<offset>]|<memaddr>\n"
 #endif
 #ifdef CONFIG_UPROBE_EVENTS
-	"\t    place: <path>:<offset>\n"
+  "   place (uprobe): <path>:<offset>[(ref_ctr_offset)]\n"
 #endif
 	"\t     args: <name>=fetcharg[:type]\n"
 	"\t fetcharg: %<register>, @<address>, @<symbol>[+|-<offset>],\n"
-- 
1.8.3.1