tracing: Fix a race in function profile

While we are reading trace_stat/functionX and someone just
disabled function_profile at that time, we can trigger this:

	divide error: 0000 [#1] PREEMPT SMP
	...
	EIP is at function_stat_show+0x90/0x230
	...

This fix just takes the ftrace_profile_lock and checks if
rec->counter is 0. If it's 0, we know the profile buffer
has been reset.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: stable@kernel.org
LKML-Reference: <4C723644.4040708@cn.fujitsu.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 11 insertions, 4 deletions

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 0d88ce9b9fb8..7cb1f45a1de1 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -381,12 +381,19 @@ static int function_stat_show(struct seq_file *m, void *v)
 {
         struct ftrace_profile *rec = v;
         char str[KSYM_SYMBOL_LEN];
+        int ret = 0;
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
-        static DEFINE_MUTEX(mutex);
         static struct trace_seq s;
         unsigned long long avg;
         unsigned long long stddev;
 #endif
+        mutex_lock(&ftrace_profile_lock);
+
+        /* we raced with function_profile_reset() */
+        if (unlikely(rec->counter == 0)) {
+                ret = -EBUSY;
+                goto out;
+        }
 
         kallsyms_lookup(rec->ip, NULL, NULL, NULL, str);
         seq_printf(m, "  %-30.30s  %10lu", str, rec->counter);
@@ -408,7 +415,6 @@ static int function_stat_show(struct seq_file *m, void *v)
                 do_div(stddev, (rec->counter - 1) * 1000);
         }
 
-        mutex_lock(&mutex);
         trace_seq_init(&s);
         trace_print_graph_duration(rec->time, &s);
         trace_seq_puts(&s, "    ");
@@ -416,11 +422,12 @@ static int function_stat_show(struct seq_file *m, void *v)
         trace_seq_puts(&s, "    ");
         trace_print_graph_duration(stddev, &s);
         trace_print_seq(m, &s);
-        mutex_unlock(&mutex);
 #endif
         seq_putc(m, '\n');
+out:
+        mutex_unlock(&ftrace_profile_lock);
 
-        return 0;
+        return ret;
 }
 
 static void ftrace_profile_reset(struct ftrace_profile_stat *stat)