sample-trace-array: Fix sleeping function called from invalid context

BUG: sleeping function called from invalid context at kernel/locking/mutex.c:935
 in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 0, name: swapper/5
 1 lock held by swapper/5/0:
  #0: ffff80001002bd90 (samples/ftrace/sample-trace-array.c:38){+.-.}-{0:0}, at: call_timer_fn+0x8/0x3e0
 CPU: 5 PID: 0 Comm: swapper/5 Not tainted 5.7.0+ #8
 Hardware name: QEMU QEMU Virtual Machine, BIOS 0.0.0 02/06/2015
 Call trace:
  dump_backtrace+0x0/0x1a0
  show_stack+0x20/0x30
  dump_stack+0xe4/0x150
  ___might_sleep+0x160/0x200
  __might_sleep+0x58/0x90
  __mutex_lock+0x64/0x948
  mutex_lock_nested+0x3c/0x58
  __ftrace_set_clr_event+0x44/0x88
  trace_array_set_clr_event+0x24/0x38
  mytimer_handler+0x34/0x40 [sample_trace_array]

mutex_lock() will be called in interrupt context, using workqueue to fix it.

Link: https://lkml.kernel.org/r/20200610011244.2209486-1-wangkefeng.wang@huawei.com

Cc: stable@vger.kernel.org
Fixes: 89ed4249 ("tracing: Sample module to demonstrate kernel access to Ftrace instances.")
Reviewed-by: Divya Indi <divya.indi@oracle.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

samples/ftrace/sample-trace-array.c

@@ -6,6 +6,7 @@
 #include <linux/timer.h>
 #include <linux/err.h>
 #include <linux/jiffies.h>
+#include <linux/workqueue.h>
 
 /*
  * Any file that uses trace points, must include the header.
@@ -20,6 +21,16 @@ struct trace_array *tr;
 static void mytimer_handler(struct timer_list *unused);
 static struct task_struct *simple_tsk;
 
+static void trace_work_fn(struct work_struct *work)
+{
+	/*
+	 * Disable tracing for event "sample_event".
+	 */
+	trace_array_set_clr_event(tr, "sample-subsystem", "sample_event",
+			false);
+}
+static DECLARE_WORK(trace_work, trace_work_fn);
+
 /*
  * mytimer: Timer setup to disable tracing for event "sample_event". This
  * timer is only for the purposes of the sample module to demonstrate access of
@@ -29,11 +40,7 @@ static DEFINE_TIMER(mytimer, mytimer_handler);
 
 static void mytimer_handler(struct timer_list *unused)
 {
-	/*
-	 * Disable tracing for event "sample_event".
-	 */
-	trace_array_set_clr_event(tr, "sample-subsystem", "sample_event",
-			false);
+	schedule_work(&trace_work);
 }
 
 static void simple_thread_func(int count)
@@ -76,6 +83,7 @@ static int simple_thread(void *arg)
 		simple_thread_func(count++);
 
 	del_timer(&mytimer);
+	cancel_work_sync(&trace_work);
 
 	/*
 	 * trace_array_put() decrements the reference counter associated with