tracing: Use same local variable when resetting the ring buffer

In the ftrace code that resets the ring buffer it references the
buffer with a local variable, but then uses the tr->buffer as the
parameter to reset. If the wakeup tracer is running, which can
switch the tr->buffer with the max saved buffer, this can break
the requirement of disabling the buffer before the reset.

   buffer = tr->buffer;
   ring_buffer_record_disable(buffer);
   synchronize_sched();
   __tracing_reset(tr->buffer, cpu);

If the tr->buffer is swapped, then the reset is not happening to the
buffer that was disabled. This will cause the ring buffer to fail.

Found with Li Zefan's ftrace_stress_test.

Cc: stable@kernel.org
Reported-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -840,10 +840,10 @@ void unregister_tracer(struct tracer *type)
 	mutex_unlock(&trace_types_lock);
 }
 
-static void __tracing_reset(struct trace_array *tr, int cpu)
+static void __tracing_reset(struct ring_buffer *buffer, int cpu)
 {
 	ftrace_disable_cpu();
-	ring_buffer_reset_cpu(tr->buffer, cpu);
+	ring_buffer_reset_cpu(buffer, cpu);
 	ftrace_enable_cpu();
 }
 
@@ -855,7 +855,7 @@ void tracing_reset(struct trace_array *tr, int cpu)
 
 	/* Make sure all commits have finished */
 	synchronize_sched();
-	__tracing_reset(tr, cpu);
+	__tracing_reset(buffer, cpu);
 
 	ring_buffer_record_enable(buffer);
 }
@@ -873,7 +873,7 @@ void tracing_reset_online_cpus(struct trace_array *tr)
 	tr->time_start = ftrace_now(tr->cpu);
 
 	for_each_online_cpu(cpu)
-		__tracing_reset(tr, cpu);
+		__tracing_reset(buffer, cpu);
 
 	ring_buffer_record_enable(buffer);
 }