ring-buffer: Add verifier for using ring_buffer_event_time_stamp()

The ring_buffer_event_time_stamp() must be only called by an event that has
not been committed yet, and is on the buffer that is passed in. This was
used to help debug converting the histogram logic over to using the new
time stamp code, and was proven to be very useful.

Add a verifier that can check that this is the case, and extra WARN_ONs to
catch unexpected use cases.

Link: https://lkml.kernel.org/r/20210316164113.987294354@goodmis.orgReviewed-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/ring_buffer.c

@@ -742,6 +742,48 @@ static bool rb_time_cmpxchg(rb_time_t *t, u64 expect, u64 set)
 }
 #endif
 
+/*
+ * Enable this to make sure that the event passed to
+ * ring_buffer_event_time_stamp() is not committed and also
+ * is on the buffer that it passed in.
+ */
+//#define RB_VERIFY_EVENT
+#ifdef RB_VERIFY_EVENT
+static struct list_head *rb_list_head(struct list_head *list);
+static void verify_event(struct ring_buffer_per_cpu *cpu_buffer,
+			 void *event)
+{
+	struct buffer_page *page = cpu_buffer->commit_page;
+	struct buffer_page *tail_page = READ_ONCE(cpu_buffer->tail_page);
+	struct list_head *next;
+	long commit, write;
+	unsigned long addr = (unsigned long)event;
+	bool done = false;
+	int stop = 0;
+
+	/* Make sure the event exists and is not committed yet */
+	do {
+		if (page == tail_page || WARN_ON_ONCE(stop++ > 100))
+			done = true;
+		commit = local_read(&page->page->commit);
+		write = local_read(&page->write);
+		if (addr >= (unsigned long)&page->page->data[commit] &&
+		    addr < (unsigned long)&page->page->data[write])
+			return;
+
+		next = rb_list_head(page->list.next);
+		page = list_entry(next, struct buffer_page, list);
+	} while (!done);
+	WARN_ON_ONCE(1);
+}
+#else
+static inline void verify_event(struct ring_buffer_per_cpu *cpu_buffer,
+			 void *event)
+{
+}
+#endif
+
+
 static inline u64 rb_time_stamp(struct trace_buffer *buffer);
 
 /**
@@ -772,13 +814,19 @@ u64 ring_buffer_event_time_stamp(struct trace_buffer *buffer,
 	if (event->type_len == RINGBUF_TYPE_TIME_STAMP)
 		return rb_event_time_stamp(event);
 
+	nest = local_read(&cpu_buffer->committing);
+	verify_event(cpu_buffer, event);
+	if (WARN_ON_ONCE(!nest))
+		goto fail;
+
 	/* Read the current saved nesting level time stamp */
-	nest = local_read(&cpu_buffer->committing) - 1;
-	if (likely(nest < MAX_NEST))
+	if (likely(--nest < MAX_NEST))
 		return cpu_buffer->event_stamp[nest];
 
-	WARN_ON_ONCE(1);
+	/* Shouldn't happen, warn if it does */
+	WARN_ONCE(1, "nest (%d) greater than max", nest);
 
+ fail:
 	/* Can only fail on 32 bit */
 	if (!rb_time_read(&cpu_buffer->write_stamp, &ts))
 		/* Screw it, just read the current time */
@@ -2750,7 +2798,7 @@ rb_update_event(struct ring_buffer_per_cpu *cpu_buffer,
 	u64 delta = info->delta;
 	unsigned int nest = local_read(&cpu_buffer->committing) - 1;
 
-	if (nest < MAX_NEST)
+	if (!WARN_ON_ONCE(nest >= MAX_NEST))
 		cpu_buffer->event_stamp[nest] = info->ts;
 
 	/*