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SubjectRe: [PATCH v2 35/40] tracing: Reverse the order event_mutex/trace_types_lock are taken
On Tue,  5 Sep 2017 16:57:47 -0500
Tom Zanussi <tom.zanussi@linux.intel.com> wrote:

> Change the order event_mutex and trace_types_lock are taken, to avoid
> circular dependencies and lockdep spew.
>
> Changing the order shouldn't matter to any current code, but does to
> anything that takes the event_mutex first and then trace_types_lock.
> This is the case when calling tracing_set_clock from inside an event
> command, which already holds the event_mutex.

This is a very scary patch. I'll apply it and run a bunch of tests with
lockdep enabled. Let's see what blows up (or not).

-- Steve

>
> Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
> ---
> kernel/trace/trace_events.c | 16 ++++++++--------
> 1 file changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> index c93540c..889802c 100644
> --- a/kernel/trace/trace_events.c
> +++ b/kernel/trace/trace_events.c
> @@ -1406,8 +1406,8 @@ static int subsystem_open(struct inode *inode, struct file *filp)
> return -ENODEV;
>
> /* Make sure the system still exists */
> - mutex_lock(&trace_types_lock);
> mutex_lock(&event_mutex);
> + mutex_lock(&trace_types_lock);
> list_for_each_entry(tr, &ftrace_trace_arrays, list) {
> list_for_each_entry(dir, &tr->systems, list) {
> if (dir == inode->i_private) {
> @@ -1421,8 +1421,8 @@ static int subsystem_open(struct inode *inode, struct file *filp)
> }
> }
> exit_loop:
> - mutex_unlock(&event_mutex);
> mutex_unlock(&trace_types_lock);
> + mutex_unlock(&event_mutex);
>
> if (!system)
> return -ENODEV;
> @@ -2294,15 +2294,15 @@ void trace_event_eval_update(struct trace_eval_map **map, int len)
> int trace_add_event_call(struct trace_event_call *call)
> {
> int ret;
> - mutex_lock(&trace_types_lock);
> mutex_lock(&event_mutex);
> + mutex_lock(&trace_types_lock);
>
> ret = __register_event(call, NULL);
> if (ret >= 0)
> __add_event_to_tracers(call);
>
> - mutex_unlock(&event_mutex);
> mutex_unlock(&trace_types_lock);
> + mutex_unlock(&event_mutex);
> return ret;
> }
>
> @@ -2356,13 +2356,13 @@ int trace_remove_event_call(struct trace_event_call *call)
> {
> int ret;
>
> - mutex_lock(&trace_types_lock);
> mutex_lock(&event_mutex);
> + mutex_lock(&trace_types_lock);
> down_write(&trace_event_sem);
> ret = probe_remove_event_call(call);
> up_write(&trace_event_sem);
> - mutex_unlock(&event_mutex);
> mutex_unlock(&trace_types_lock);
> + mutex_unlock(&event_mutex);
>
> return ret;
> }
> @@ -2424,8 +2424,8 @@ static int trace_module_notify(struct notifier_block *self,
> {
> struct module *mod = data;
>
> - mutex_lock(&trace_types_lock);
> mutex_lock(&event_mutex);
> + mutex_lock(&trace_types_lock);
> switch (val) {
> case MODULE_STATE_COMING:
> trace_module_add_events(mod);
> @@ -2434,8 +2434,8 @@ static int trace_module_notify(struct notifier_block *self,
> trace_module_remove_events(mod);
> break;
> }
> - mutex_unlock(&event_mutex);
> mutex_unlock(&trace_types_lock);
> + mutex_unlock(&event_mutex);
>
> return 0;
> }

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