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

[KOSAKI Motohiro]

Commit-ID:  10dd3ebe213c31bff14b4dae3c5d32a76b1fad7c
Gitweb:     http://git.kernel.org/tip/10dd3ebe213c31bff14b4dae3c5d32a76b1fad7c
Author:     "KOSAKI Motohiro" <kosaki.motohiro@xxxxxxxxxxxxxx>
AuthorDate: Fri, 6 Mar 2009 15:29:04 +0900
Commit:     Ingo Molnar <mingo@xxxxxxx>
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@xxxxxxxxxxxxxx>
Reviewed-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
Cc: Steven Rostedt <srostedt@xxxxxxxxxx>
LKML-Reference: <20090306151155.0778.A69D9226@xxxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>


---
 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 */
--
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