ftrace: show debugging information when panic_on_warn set

When an anomaly is detected in the function call modification
code, ftrace_bug() is called to disable function tracing as well
as give some warn and information that may help debug the problem.

But currently, we call FTRACE_WARN_ON_ONCE() first in ftrace_bug(),
so when panic_on_warn is set, we can't see the debugging information
here. Call FTRACE_WARN_ON_ONCE() at the end of ftrace_bug() to ensure
that the debugging information is displayed first.

after this patch, the dmesg looks like:

	------------[ ftrace bug ]------------
	ftrace failed to modify
	[<ffff800010081004>] bcm2835_handle_irq+0x4/0x58
	 actual:   1f:20:03:d5
	Setting ftrace call site to call ftrace function
	ftrace record flags: 80000001
	 (1)
	 expected tramp: ffff80001009d6f0
	------------[ cut here ]------------
	WARNING: CPU: 2 PID: 1635 at kernel/trace/ftrace.c:2078 ftrace_bug+0x204/0x238
	Kernel panic - not syncing: panic_on_warn set ...
	CPU: 2 PID: 1635 Comm: sh Not tainted 5.7.0-rc5-00033-gb922183867f5 #14
	Hardware name: linux,dummy-virt (DT)
	Call trace:
	 dump_backtrace+0x0/0x1b0
	 show_stack+0x20/0x30
	 dump_stack+0xc0/0x10c
	 panic+0x16c/0x368
	 __warn+0x120/0x160
	 report_bug+0xc8/0x160
	 bug_handler+0x28/0x98
	 brk_handler+0x70/0xd0
	 do_debug_exception+0xcc/0x1ac
	 el1_sync_handler+0xe4/0x120
	 el1_sync+0x7c/0x100
	 ftrace_bug+0x204/0x238

Link: https://lkml.kernel.org/r/20200515100828.7091-1-cj.chengjian@huawei.com

Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/ftrace.c

@@ -2027,14 +2027,14 @@ void ftrace_bug(int failed, struct dyn_ftrace *rec)
 {
 	unsigned long ip = rec ? rec->ip : 0;
 
+	pr_info("------------[ ftrace bug ]------------\n");
+
 	switch (failed) {
 	case -EFAULT:
-		FTRACE_WARN_ON_ONCE(1);
 		pr_info("ftrace faulted on modifying ");
 		print_ip_sym(ip);
 		break;
 	case -EINVAL:
-		FTRACE_WARN_ON_ONCE(1);
 		pr_info("ftrace failed to modify ");
 		print_ip_sym(ip);
 		print_ip_ins(" actual:   ", (unsigned char *)ip);
@@ -2045,12 +2045,10 @@ void ftrace_bug(int failed, struct dyn_ftrace *rec)
 		}
 		break;
 	case -EPERM:
-		FTRACE_WARN_ON_ONCE(1);
 		pr_info("ftrace faulted on writing ");
 		print_ip_sym(ip);
 		break;
 	default:
-		FTRACE_WARN_ON_ONCE(1);
 		pr_info("ftrace faulted on unknown error ");
 		print_ip_sym(ip);
 	}
@@ -2077,6 +2075,8 @@ void ftrace_bug(int failed, struct dyn_ftrace *rec)
 		ip = ftrace_get_addr_curr(rec);
 		pr_cont("\n expected tramp: %lx\n", ip);
 	}
+
+	FTRACE_WARN_ON_ONCE(1);
 }
 
 static int ftrace_check_record(struct dyn_ftrace *rec, bool enable, bool update)