[bug report] x86/ftrace: Make function graph use ftrace directly

[Dan Carpenter <dan.carpenter@linaro.org>]

Hello Steven Rostedt (VMware),

The patch 0c0593b45c9b: "x86/ftrace: Make function graph use ftrace
directly" from Oct 8, 2021, leads to the following Smatch static
checker warning:

	kernel/trace/trace_selftest.c:769 trace_graph_entry_watchdog()
	warn: sleeping in atomic context

kernel/trace/trace_selftest.c
    765 static int trace_graph_entry_watchdog(struct ftrace_graph_ent *trace)
    766 {
    767         /* This is harmlessly racy, we want to approximately detect a hang */
    768         if (unlikely(++graph_hang_thresh > GRAPH_MAX_FUNC_TEST)) {
--> 769                 ftrace_graph_stop();

This is a sleeping function.

    770                 printk(KERN_WARNING "BUG: Function graph tracer hang!\n");
    771                 if (ftrace_dump_on_oops) {
    772                         ftrace_dump(DUMP_ALL);
    773                         /* ftrace_dump() disables tracing */
    774                         tracing_on();
    775                 }
    776                 return 0;
    777         }
    778 
    779         return trace_graph_entry(trace);
    780 }

The call tree is:

prepare_ftrace_return() <- disables preempt
-> function_graph_enter()
   -> trace_graph_entry_watchdog()

The ftrace_test_recursion_trylock() function disables preemption.  The
trace_graph_entry_watchdog() function is called as a pointer so it's
the ftrace_graph_entry() call.

regards,
dan carpenter