kprobes: Inverse taking of module_mutex with kprobe_mutex

Currently module_mutex is taken before kprobe_mutex, but this
can cause issues when we have kprobes register ftrace, as the ftrace
mutex is taken before enabling a tracepoint, which currently takes
the module mutex.

If module_mutex is taken before kprobe_mutex, then we can not
have kprobes use the ftrace infrastructure.

There seems to be no reason that the kprobe_mutex can't be taken
before the module_mutex. Running lockdep shows that it is safe
among the kernels I've run.

Link: http://lkml.kernel.org/r/20120605102814.27845.21047.stgit@localhost.localdomain

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Cc: "Frank Ch. Eigler" <fche@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index c62b8546cc90..7a8a1222c7b1 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -561,9 +561,9 @@ static __kprobes void kprobe_optimizer(struct work_struct *work)
 {
         LIST_HEAD(free_list);
 
+        mutex_lock(&kprobe_mutex);
         /* Lock modules while optimizing kprobes */
         mutex_lock(&module_mutex);
-        mutex_lock(&kprobe_mutex);
 
         /*
          * Step 1: Unoptimize kprobes and collect cleaned (unused and disarmed)
@@ -586,8 +586,8 @@ static __kprobes void kprobe_optimizer(struct work_struct *work)
         /* Step 4: Free cleaned kprobes after quiesence period */
         do_free_cleaned_kprobes(&free_list);
 
-        mutex_unlock(&kprobe_mutex);
         mutex_unlock(&module_mutex);
+        mutex_unlock(&kprobe_mutex);
 
         /* Step 5: Kick optimizer again if needed */
         if (!list_empty(&optimizing_list) || !list_empty(&unoptimizing_list))