Linux Kernel Markers: support multiple probes

RCU style multiple probes support for the Linux Kernel Markers.  Common case
(one probe) is still fast and does not require dynamic allocation or a
supplementary pointer dereference on the fast path.

- Move preempt disable from the marker site to the callback.

Since we now have an internal callback, move the preempt disable/enable to the
callback instead of the marker site.

Since the callback change is done asynchronously (passing from a handler that
supports arguments to a handler that does not setup the arguments is no
arguments are passed), we can safely update it even if it is outside the
preempt disable section.

- Move probe arm to probe connection. Now, a connected probe is automatically
  armed.

Remove MARK_MAX_FORMAT_LEN, unused.

This patch modifies the Linux Kernel Markers API : it removes the probe
"arm/disarm" and changes the probe function prototype : it now expects a
va_list * instead of a "...".

If we want to have more than one probe connected to a marker at a given
time (LTTng, or blktrace, ssytemtap) then we need this patch. Without it,
connecting a second probe handler to a marker will fail.

It allow us, for instance, to do interesting combinations :

Do standard tracing with LTTng and, eventually, to compute statistics
with SystemTAP, or to have a special trigger on an event that would call
a systemtap script which would stop flight recorder tracing.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Mike Mason <mmlnx@us.ibm.com>
Cc: Dipankar Sarma <dipankar@in.ibm.com>
Cc: David Smith <dsmith@redhat.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Cc: "Frank Ch. Eigler" <fche@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 37 insertions, 24 deletions

diff --git a/include/linux/marker.h b/include/linux/marker.h
index 5f36cf946b..b5f95637f2 100644
--- a/include/linux/marker.h
+++ b/include/linux/marker.h
@@ -19,16 +19,23 @@ struct marker;
 
 /**
  * marker_probe_func - Type of a marker probe function
- * @mdata: pointer of type struct marker
- * @private_data: caller site private data
+ * @probe_private: probe private data
+ * @call_private: call site private data
  * @fmt: format string
- * @...: variable argument list
+ * @args: variable argument list pointer. Use a pointer to overcome C's
+ *        inability to pass this around as a pointer in a portable manner in
+ *        the callee otherwise.
  *
  * Type of marker probe functions. They receive the mdata and need to parse the
  * format string to recover the variable argument list.
  */
-typedef void marker_probe_func(const struct marker *mdata,
-        void *private_data, const char *fmt, ...);
+typedef void marker_probe_func(void *probe_private, void *call_private,
+                const char *fmt, va_list *args);
+
+struct marker_probe_closure {
+        marker_probe_func *func;        /* Callback */
+        void *probe_private;            /* Private probe data */
+};
 
 struct marker {
         const char *name;       /* Marker name */
@@ -36,8 +43,11 @@ struct marker {
                                  * variable argument list.
                                  */
         char state;             /* Marker state. */
-        marker_probe_func *call;/* Probe handler function pointer */
-        void *private;          /* Private probe data */
+        char ptype;             /* probe type : 0 : single, 1 : multi */
+        void (*call)(const struct marker *mdata,        /* Probe wrapper */
+                void *call_private, const char *fmt, ...);
+        struct marker_probe_closure single;
+        struct marker_probe_closure *multi;
 } __attribute__((aligned(8)));
 
 #ifdef CONFIG_MARKERS
@@ -49,7 +59,7 @@ struct marker {
  * not add unwanted padding between the beginning of the section and the
  * structure. Force alignment to the same alignment as the section start.
  */
-#define __trace_mark(name, call_data, format, args...)                  \
+#define __trace_mark(name, call_private, format, args...)               \
         do {                                                            \
                 static const char __mstrtab_name_##name[]               \
                 __attribute__((section("__markers_strings")))           \
@@ -60,24 +70,23 @@ struct marker {
                 static struct marker __mark_##name                      \
                 __attribute__((section("__markers"), aligned(8))) =     \
                 { __mstrtab_name_##name, __mstrtab_format_##name,       \
-                0, __mark_empty_function, NULL };                       \
+                0, 0, marker_probe_cb,                                  \
+                { __mark_empty_function, NULL}, NULL };                 \
                 __mark_check_format(format, ## args);                   \
                 if (unlikely(__mark_##name.state)) {                    \
-                        preempt_disable();                              \
                         (*__mark_##name.call)                           \
-                                (&__mark_##name, call_data,             \
+                                (&__mark_##name, call_private,          \
                                 format, ## args);                       \
-                        preempt_enable();                               \
                 }                                                       \
         } while (0)
 
 extern void marker_update_probe_range(struct marker *begin,
-        struct marker *end, struct module *probe_module, int *refcount);
+        struct marker *end);
 #else /* !CONFIG_MARKERS */
-#define __trace_mark(name, call_data, format, args...) \
+#define __trace_mark(name, call_private, format, args...) \
                 __mark_check_format(format, ## args)
 static inline void marker_update_probe_range(struct marker *begin,
-        struct marker *end, struct module *probe_module, int *refcount)
+        struct marker *end)
 { }
 #endif /* CONFIG_MARKERS */
 
@@ -92,8 +101,6 @@ static inline void marker_update_probe_range(struct marker *begin,
 #define trace_mark(name, format, args...) \
         __trace_mark(name, NULL, format, ## args)
 
-#define MARK_MAX_FORMAT_LEN     1024
-
 /**
  * MARK_NOARGS - Format string for a marker with no argument.
  */
@@ -106,24 +113,30 @@ static inline void __printf(1, 2) __mark_check_format(const char *fmt, ...)
 
 extern marker_probe_func __mark_empty_function;
 
+extern void marker_probe_cb(const struct marker *mdata,
+        void *call_private, const char *fmt, ...);
+extern void marker_probe_cb_noarg(const struct marker *mdata,
+        void *call_private, const char *fmt, ...);
+
 /*
  * Connect a probe to a marker.
  * private data pointer must be a valid allocated memory address, or NULL.
  */
 extern int marker_probe_register(const char *name, const char *format,
-                                marker_probe_func *probe, void *private);
+                                marker_probe_func *probe, void *probe_private);
 
 /*
  * Returns the private data given to marker_probe_register.
  */
-extern void *marker_probe_unregister(const char *name);
+extern int marker_probe_unregister(const char *name,
+        marker_probe_func *probe, void *probe_private);
 /*
  * Unregister a marker by providing the registered private data.
  */
-extern void *marker_probe_unregister_private_data(void *private);
+extern int marker_probe_unregister_private_data(marker_probe_func *probe,
+        void *probe_private);
 
-extern int marker_arm(const char *name);
-extern int marker_disarm(const char *name);
-extern void *marker_get_private_data(const char *name);
+extern void *marker_get_private_data(const char *name, marker_probe_func *probe,
+        int num);
 
 #endif
diff --git a/include/linux/module.h b/include/linux/module.h
index ac28e8761e..330bec08c2 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -465,7 +465,7 @@ int unregister_module_notifier(struct notifier_block * nb);
 
 extern void print_modules(void);
 
-extern void module_update_markers(struct module *probe_module, int *refcount);
+extern void module_update_markers(void);
 
 #else /* !CONFIG_MODULES... */
 #define EXPORT_SYMBOL(sym)