tracing: Fix race in snapshot swapping

[deliverable/linux.git] / kernel / trace / trace.c

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index c2e2c23103742e3a3e53cf7b318aa0c804f5e1a8..53df2839bb9396bbb01f63241189400917c5778d 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -704,7 +704,7 @@ __update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu)
 void
 update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu)
 {
-       struct ring_buffer *buf = tr->buffer;
+       struct ring_buffer *buf;
 
        if (trace_stop_count)
                return;
@@ -719,6 +719,7 @@ update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu)
 
        arch_spin_lock(&ftrace_max_lock);
 
+       buf = tr->buffer;
        tr->buffer = max_tr.buffer;
        max_tr.buffer = buf;
 
@@ -2400,6 +2401,27 @@ static void test_ftrace_alive(struct seq_file *m)
        seq_printf(m, "#          MAY BE MISSING FUNCTION EVENTS\n");
 }
 
+#ifdef CONFIG_TRACER_MAX_TRACE
+static void print_snapshot_help(struct seq_file *m, struct trace_iterator *iter)
+{
+       if (iter->trace->allocated_snapshot)
+               seq_printf(m, "#\n# * Snapshot is allocated *\n#\n");
+       else
+               seq_printf(m, "#\n# * Snapshot is freed *\n#\n");
+
+       seq_printf(m, "# Snapshot commands:\n");
+       seq_printf(m, "# echo 0 > snapshot : Clears and frees snapshot buffer\n");
+       seq_printf(m, "# echo 1 > snapshot : Allocates snapshot buffer, if not already allocated.\n");
+       seq_printf(m, "#                      Takes a snapshot of the main buffer.\n");
+       seq_printf(m, "# echo 2 > snapshot : Clears snapshot buffer (but does not allocate)\n");
+       seq_printf(m, "#                      (Doesn't have to be '2' works with any number that\n");
+       seq_printf(m, "#                       is not a '0' or '1')\n");
+}
+#else
+/* Should never be called */
+static inline void print_snapshot_help(struct seq_file *m, struct trace_iterator *iter) { }
+#endif
+
 static int s_show(struct seq_file *m, void *v)
 {
        struct trace_iterator *iter = v;
@@ -2411,7 +2433,9 @@ static int s_show(struct seq_file *m, void *v)
                        seq_puts(m, "#\n");
                        test_ftrace_alive(m);
                }
-               if (iter->trace && iter->trace->print_header)
+               if (iter->snapshot && trace_empty(iter))
+                       print_snapshot_help(m, iter);
+               else if (iter->trace && iter->trace->print_header)
                        iter->trace->print_header(m);
                else
                        trace_default_header(m);
@@ -4144,8 +4168,6 @@ tracing_snapshot_write(struct file *filp, const char __user *ubuf, size_t cnt,
        default:
                if (current_trace->allocated_snapshot)
                        tracing_reset_online_cpus(&max_tr);
-               else
-                       ret = -EINVAL;
                break;
        }