[tip:tracing/ftrace] tracing: fix deadlock when setting set_ftrace_pid

KOSAKI Motohiro
2009-03-06T11:10:14Z

Commit-ID: 10dd3ebe213c31bff14b4dae3c5d32a76b1fad7c
Gitweb: http://git.kernel.org/tip/10dd3ebe213c31bff14b4dae3c5d32a76b1fad7c
Author: "KOSAKI Motohiro" <kosaki.motohiro@jp.fujitsu.com>
AuthorDate: Fri, 6 Mar 2009 15:29:04 +0900
Commit: Ingo Molnar <mingo@elte.hu>
CommitDate: Fri, 6 Mar 2009 12:07:38 +0100

tracing: fix deadlock when setting set_ftrace_pid

Impact: fix deadlock while using set_ftrace_pid

Reproducer:

# cd /sys/kernel/debug/tracing
# echo $$ > set_ftrace_pid

then, console becomes hung.

Details:

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 always deadlocks when ftrace_pid_write() is called.

In past days, ftrace_pid_write() used ftrace_start_lock, but
commit e6ea44e9b4c12325337cd1c06103cd515a1c02b2 consolidated
ftrace_start_lock to ftrace_lock.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Steven Rostedt <srostedt@redhat.com>
LKML-Reference: <20090306151155.0778.A69D9226@jp.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

---
kernel/trace/ftrace.c | 7 +------
1 files changed, 1 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index d7a06a0..d33d306 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 */