linux-kernel - [PATCH] kgdboc: disable the console lock when in kgdb

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Date:   Thu, 25 Jul 2019 11:35:51 -0700
From:   Douglas Anderson <dianders@...omium.org>
To:     Jason Wessel <jason.wessel@...driver.com>,
        Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Daniel Vetter <daniel.vetter@...el.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Sam Ravnborg <sam@...nborg.org>,
        Douglas Anderson <dianders@...omium.org>,
        Jiri Slaby <jslaby@...e.com>,
        kgdb-bugreport@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
        linux-serial@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: [PATCH] kgdboc: disable the console lock when in kgdb

After commit ddde3c18b700 ("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: ddde3c18b700 ("vt: More locking checks")
Cc: Daniel Vetter <daniel.vetter@...el.com>
Signed-off-by: Douglas Anderson <dianders@...omium.org>
---

 drivers/tty/serial/kgdboc.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
index bfe5e9e034ec..c7d51b51898f 100644
--- a/drivers/tty/serial/kgdboc.c
+++ b/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);
-- 
2.22.0.709.g102302147b-goog