kgdboc: disable the console lock when in kgdb

After commit ddde3c1 ("vt: More locking checks") kdb / kgdb has
become useless because my console is filled with spews of:

WARNING: CPU: 0 PID: 0 at .../drivers/tty/vt/vt.c:3846 con_is_visible+0x50/0x74
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.3.0-rc1+ #48
Hardware name: Rockchip (Device Tree)
Backtrace:
[<c020ce9c>] (dump_backtrace) from [<c020d188>] (show_stack+0x20/0x24)
[<c020d168>] (show_stack) from [<c0a8fc14>] (dump_stack+0xb0/0xd0)
[<c0a8fb64>] (dump_stack) from [<c0232c58>] (__warn+0xec/0x11c)
[<c0232b6c>] (__warn) from [<c0232dc4>] (warn_slowpath_null+0x4c/0x58)
[<c0232d78>] (warn_slowpath_null) from [<c06338a0>] (con_is_visible+0x50/0x74)
[<c0633850>] (con_is_visible) from [<c0634078>] (con_scroll+0x108/0x1ac)
[<c0633f70>] (con_scroll) from [<c0634160>] (lf+0x44/0x88)
[<c063411c>] (lf) from [<c06363ec>] (vt_console_print+0x1a4/0x2bc)
[<c0636248>] (vt_console_print) from [<c02f628c>] (vkdb_printf+0x420/0x8a4)
[<c02f5e6c>] (vkdb_printf) from [<c02f6754>] (kdb_printf+0x44/0x60)
[<c02f6714>] (kdb_printf) from [<c02fa6f4>] (kdb_main_loop+0xf4/0x6e0)
[<c02fa600>] (kdb_main_loop) from [<c02fd5f0>] (kdb_stub+0x268/0x398)
[<c02fd388>] (kdb_stub) from [<c02f3ba0>] (kgdb_cpu_enter+0x1f8/0x674)
[<c02f39a8>] (kgdb_cpu_enter) from [<c02f4330>] (kgdb_handle_exception+0x1c4/0x1fc)
[<c02f416c>] (kgdb_handle_exception) from [<c0210fe0>] (kgdb_compiled_brk_fn+0x30/0x3c)
[<c0210fb0>] (kgdb_compiled_brk_fn) from [<c020d7ac>] (do_undefinstr+0x180/0x1a0)
[<c020d62c>] (do_undefinstr) from [<c0201b44>] (__und_svc_finish+0x0/0x3c)
...
[<c02f3224>] (kgdb_breakpoint) from [<c02f3310>] (sysrq_handle_dbg+0x58/0x6c)
[<c02f32b8>] (sysrq_handle_dbg) from [<c062abf0>] (__handle_sysrq+0xac/0x154)

Let's disable this warning when we're in kgdb to avoid the spew.  The
whole system is stopped when we're in kgdb so we can't exactly wait
for someone else to drop the lock.  Presumably the best we can do is
to disable the warning and hope for the best.

Fixes: ddde3c1 ("vt: More locking checks")
Cc: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20190725183551.169208-1-dianders@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/kgdboc.c

@@ -277,10 +277,14 @@ static void kgdboc_pre_exp_handler(void)
 	/* Increment the module count when the debugger is active */
 	if (!kgdb_connected)
 		try_module_get(THIS_MODULE);
+
+	atomic_inc(&ignore_console_lock_warning);
 }
 
 static void kgdboc_post_exp_handler(void)
 {
+	atomic_dec(&ignore_console_lock_warning);
+
 	/* decrement the module count when the debugger detaches */
 	if (!kgdb_connected)
 		module_put(THIS_MODULE);