[PATCH for tip][bugfix] ftrace: fix deadlock when setting set_ftrace_pid

KOSAKI Motohiro
2009-03-06T06:29:53Z

Impact: fix deadlock bug

reproduce way:
# cd /sys/kernel/debug/tracing
# echo $$ > set_ftrace_pid

then, console become hangup.

detail:
when writing set_ftracepid, kernel callstack is following

ftrace_pid_write()
mutex_lock(&ftrace_lock);
ftrace_update_pid_func()
mutex_lock(&ftrace_lock);
mutex_unlock(&ftrace_lock);
mutex_unlock(&ftrace_lock);

then, system alaways deadlock when ftrace_pid_write() is called.

In past days, ftrace_pid_write() use ftrace_start_lock, but
commit e6ea44e9b4c12325337cd1c06103cd515a1c02b2 consolidate
ftrace_start_lock to ftrace_lock.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 5a3a06b..0d3b393 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -218,10 +218,8 @@ static void ftrace_update_pid_func(void)
{
ftrace_func_t func;

- mutex_lock(&ftrace_lock);
-
if (ftrace_trace_function == ftrace_stub)
- goto out;
+ return;

func = ftrace_trace_function;

@@ -238,9 +236,6 @@ static void ftrace_update_pid_func(void)
#else
__ftrace_trace_function = func;
#endif
-
- out:
- mutex_unlock(&ftrace_lock);
}

/* set when tracing only a pid */