From e0d3da982c96aeddc1bbf1cf9469dbb9ebdca657 Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Wed, 23 Nov 2022 21:18:57 +0100
Subject: [PATCH] Bluetooth: hci_qca: Fix the teardown problem for real
Git-commit: e0d3da982c96aeddc1bbf1cf9469dbb9ebdca657
References: git-fixes
Patch-mainline: v6.2-rc1

While discussing solutions for the teardown problem which results from
circular dependencies between timers and workqueues, where timers schedule
work from their timer callback and workqueues arm the timers from work
items, it was discovered that the recent fix to the QCA code is incorrect.

That commit fixes the obvious problem of using del_timer() instead of
del_timer_sync() and reorders the teardown calls to

   destroy_workqueue(wq);
   del_timer_sync(t);

This makes it less likely to explode, but it's still broken:

   destroy_workqueue(wq);
   /* After this point @wq cannot be touched anymore */

   ---> timer expires
         queue_work(wq) <---- Results in a NULL pointer dereference
			      deep in the work queue core code.
   del_timer_sync(t);

Use the new timer_shutdown_sync() function to ensure that the timers are
disarmed, no timer callbacks are running and the timers cannot be armed
again. This restores the original teardown sequence:

   timer_shutdown_sync(t);
   destroy_workqueue(wq);

which is now correct because the timer core silently ignores potential
rearming attempts which can happen when destroy_workqueue() drains pending
work before mopping up the workqueue.

Fixes: 72ef98445aca ("Bluetooth: hci_qca: Use del_timer_sync() before freeing")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Acked-by: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Link: https://lore.kernel.org/all/87iljhsftt.ffs@tglx
Link: https://lore.kernel.org/r/20221123201625.435907114@linutronix.de
Signed-off-by: Oliver Neukum <oneukum@suse.com>
---
 drivers/bluetooth/hci_qca.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/drivers/bluetooth/hci_qca.c
+++ b/drivers/bluetooth/hci_qca.c
@@ -524,9 +524,15 @@ static int qca_close(struct hci_uart *hu
 
 	skb_queue_purge(&qca->tx_wait_q);
 	skb_queue_purge(&qca->txq);
-	destroy_workqueue(qca->workqueue);
+	/*
+	 * Shut the timers down so they can't be rearmed when
+	 * destroy_workqueue() drains pending work which in turn might try
+	 * to arm a timer.  After shutdown rearm attempts are silently
+	 * ignored by the timer core code.
+	 */
 	del_timer_sync(&qca->tx_idle_timer);
 	del_timer_sync(&qca->wake_retrans_timer);
+	destroy_workqueue(qca->workqueue);
 	qca->hu = NULL;
 
 	kfree_skb(qca->rx_skb);