[PATCH v1] kdb: Fix the deadlock issue in KDB debugging.

[<liu.yec@xxxxxxx>]

From: LiuYe <liu.yeC@xxxxxxx>

Currently, if CONFIG_KDB_KEYBOARD is enabled, then kgdboc will
  attempt to use schedule_work() to provoke a keyboard reset when transitioning out
of the debugger and back to normal operation. This can cause
deadlock because schedule_work() is not NMI-safe.

example:
 BUG: spinlock lockup suspected on CPU#0, namex/10450
 lock: 0xffff881ffe823980, .magic: dead4ead, .owner: namexx/21888, .owner_cpu: 1
 ffff881741d00000 ffff881741c01000 0000000000000000 0000000000000000
 ffff881740f58e78 ffff881741cffdd0 ffffffff8147a7fc ffff881740f58f20
Call Trace:
 [<ffffffff81479e6d>] ? __schedule+0x16d/0xac0
 [<ffffffff8147a7fc>] ? schedule+0x3c/0x90
 [<ffffffff8147e71a>] ? schedule_hrtimeout_range_clock+0x10a/0x120
 [<ffffffff8147d22e>] ? mutex_unlock+0xe/0x10
 [<ffffffff811c839b>] ? ep_scan_ready_list+0x1db/0x1e0
 [<ffffffff8147e743>] ? schedule_hrtimeout_range+0x13/0x20
 [<ffffffff811c864a>] ? ep_poll+0x27a/0x3b0
 [<ffffffff8108c540>] ? wake_up_q+0x70/0x70
 [<ffffffff811c99a8>] ? SyS_epoll_wait+0xb8/0xd0
 [<ffffffff8147f296>] ? entry_SYSCALL_64_fastpath+0x12/0x75
 CPU: 0 PID: 10450 Comm: namex Tainted: G           O    4.4.65 #1
 Hardware name: Insyde Purley/Type2 - Board Product Name1, BIOS 05.21.51.0036 07/19/2019
  0000000000000000 ffff881ffe813c10 ffffffff8124e883 ffff881741c01000
  ffff881ffe823980 ffff881ffe813c38 ffffffff810a7f7f ffff881ffe823980
  000000007d2b7cd0 0000000000000001 ffff881ffe813c68 ffffffff810a80e0
  Call Trace:
  <#DB>  [<ffffffff8124e883>] dump_stack+0x85/0xc2
  [<ffffffff810a7f7f>] spin_dump+0x7f/0x100
  [<ffffffff810a80e0>] do_raw_spin_lock+0xa0/0x150
  [<ffffffff8147eb55>] _raw_spin_lock+0x15/0x20
  [<ffffffff8108c256>] try_to_wake_up+0x176/0x3d0
  [<ffffffff8108c4c5>] wake_up_process+0x15/0x20
  [<ffffffff8107b371>] insert_work+0x81/0xc0
  [<ffffffff8107b4e5>] __queue_work+0x135/0x390
  [<ffffffff8107b786>] queue_work_on+0x46/0x90
  [<ffffffff81313d28>] kgdboc_post_exp_handler+0x48/0x70
  [<ffffffff810ed488>] kgdb_cpu_enter+0x598/0x610
  [<ffffffff810ed6e2>] kgdb_handle_exception+0xf2/0x1f0
  [<ffffffff81054e21>] __kgdb_notify+0x71/0xd0
  [<ffffffff81054eb5>] kgdb_notify+0x35/0x70
  [<ffffffff81082e6a>] notifier_call_chain+0x4a/0x70
  [<ffffffff8108304d>] notify_die+0x3d/0x50
  [<ffffffff81017219>] do_int3+0x89/0x120
  [<ffffffff81480fb4>] int3+0x44/0x80

Suggested-by: daniel.thompson@xxxxxxxxxx
Signed-off-by: LiuYe <liu.yeC@xxxxxxx>
---
 drivers/tty/serial/kgdboc.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
index 7ce7bb164..161b25ecc 100644
--- a/drivers/tty/serial/kgdboc.c
+++ b/drivers/tty/serial/kgdboc.c
@@ -22,6 +22,7 @@
 #include <linux/module.h>
 #include <linux/platform_device.h>
 #include <linux/serial_core.h>
+#include <linux/irq_work.h>
 
 #define MAX_CONFIG_LEN		40
 
@@ -99,10 +100,17 @@ static void kgdboc_restore_input_helper(struct work_struct *dummy)
 
 static DECLARE_WORK(kgdboc_restore_input_work, kgdboc_restore_input_helper);
 
+static void kgdboc_queue_restore_input_helper(struct irq_work *unused)
+{
+	schedule_work(&kgdboc_restore_input_work);
+}
+
+static DEFINE_IRQ_WORK(kgdboc_restore_input_irq_work, kgdboc_queue_restore_input_helper);
+
 static void kgdboc_restore_input(void)
 {
 	if (likely(system_state == SYSTEM_RUNNING))
-		schedule_work(&kgdboc_restore_input_work);
+		irq_work_queue(&kgdboc_restore_input_irq_work);
 }
 
 static int kgdboc_register_kbd(char **cptr)
@@ -133,6 +141,7 @@ static void kgdboc_unregister_kbd(void)
 			i--;
 		}
 	}
+	irq_work_sync(&kgdboc_restore_input_irq_work);
 	flush_work(&kgdboc_restore_input_work);
 }
 #else /* ! CONFIG_KDB_KEYBOARD */
-- 
2.25.1