tracing: Only run synchronize_sched() at instance deletion time

It has been reported that boot up with FTRACE_SELFTEST enabled can take a
very long time. There can be stalls of over a minute.

This was tracked down to the synchronize_sched() called when a system call
event is disabled. As the self tests enable and disable thousands of events,
this makes the synchronize_sched() get called thousands of times.

The synchornize_sched() was added with d562aff93bfb53 "tracing: Add support
for SOFT_DISABLE to syscall events" which caused this regression (added
in 3.13-rc1).

The synchronize_sched() is to protect against the events being accessed
when a tracer instance is being deleted. When an instance is being deleted
all the events associated to it are unregistered. The synchronize_sched()
makes sure that no more users are running when it finishes.

Instead of calling synchronize_sched() for all syscall events, we only
need to call it once, after the events are unregistered and before the
instance is deleted. The event_mutex is held during this action to
prevent new users from enabling events.

Link: http://lkml.kernel.org/r/20131203124120.427b9661@gandalf.local.home

Reported-by: Petr Mladek <pmladek@suse.cz>
Acked-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Acked-by: Petr Mladek <pmladek@suse.cz>
Tested-by: Petr Mladek <pmladek@suse.cz>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

2 files changed, 3 insertions, 10 deletions

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index f919a2e21bf3..a11800ae96de 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -2314,6 +2314,9 @@ int event_trace_del_tracer(struct trace_array *tr)
         /* Disable any running events */
         __ftrace_set_clr_event_nolock(tr, NULL, NULL, NULL, 0);
 
+        /* Access to events are within rcu_read_lock_sched() */
+        synchronize_sched();
+
         down_write(&trace_event_sem);
         __trace_remove_event_dirs(tr);
         debugfs_remove_recursive(tr->event_dir);
diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
index e4b6d11bdf78..ea90eb5f6f17 100644
--- a/kernel/trace/trace_syscalls.c
+++ b/kernel/trace/trace_syscalls.c
@@ -431,11 +431,6 @@ static void unreg_event_syscall_enter(struct ftrace_event_file *file,
         if (!tr->sys_refcount_enter)
                 unregister_trace_sys_enter(ftrace_syscall_enter, tr);
         mutex_unlock(&syscall_trace_lock);
-        /*
-         * Callers expect the event to be completely disabled on
-         * return, so wait for current handlers to finish.
-         */
-        synchronize_sched();
 }
 
 static int reg_event_syscall_exit(struct ftrace_event_file *file,
@@ -474,11 +469,6 @@ static void unreg_event_syscall_exit(struct ftrace_event_file *file,
         if (!tr->sys_refcount_exit)
                 unregister_trace_sys_exit(ftrace_syscall_exit, tr);
         mutex_unlock(&syscall_trace_lock);
-        /*
-         * Callers expect the event to be completely disabled on
-         * return, so wait for current handlers to finish.
-         */
-        synchronize_sched();
 }
 
 static int __init init_syscall_trace(struct ftrace_event_call *call)