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From:   Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Date:   Tue, 15 Nov 2022 13:29:28 -0800
Message-ID: <CABBYNZ+ebSMcF0PpH47Bumm-KtQ7M_doDAGXq_Ai8z_kHcXjZA@mail.gmail.com>
Subject: Re: [patch 15/15] Bluetooth: hci_qca: Fix the teardown problem for real
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linuxfoundation.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Anna-Maria Behnsen <anna-maria@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Stephen Boyd <sboyd@kernel.org>,
        Guenter Roeck <linux@roeck-us.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Julia Lawall <Julia.Lawall@inria.fr>,
        Marcel Holtmann <marcel@holtmann.org>,
        Johan Hedberg <johan.hedberg@gmail.com>,
        linux-bluetooth@vger.kernel.org,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>, netdev@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Marc Zyngier <maz@kernel.org>
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Hi Thomas,

On Tue, Nov 15, 2022 at 12:28 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> 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>
> Cc: Marcel Holtmann <marcel@holtmann.org>
> Cc: Johan Hedberg <johan.hedberg@gmail.com>
> Cc: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
> Cc: linux-bluetooth@vger.kernel.org
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: netdev@vger.kernel.org
> Link: https://lore.kernel.org/all/87iljhsftt.ffs@tglx
> ---
>  drivers/bluetooth/hci_qca.c |   10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> --- a/drivers/bluetooth/hci_qca.c
> +++ b/drivers/bluetooth/hci_qca.c
> @@ -696,9 +696,15 @@ static int qca_close(struct hci_uart *hu
>         skb_queue_purge(&qca->tx_wait_q);
>         skb_queue_purge(&qca->txq);
>         skb_queue_purge(&qca->rx_memdump_q);
> +       /*
> +        * 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.
> +        */
> +       timer_shutdown_sync(&qca->tx_idle_timer);
> +       timer_shutdown_sync(&qca->wake_retrans_timer);
>         destroy_workqueue(qca->workqueue);
> -       del_timer_sync(&qca->tx_idle_timer);
> -       del_timer_sync(&qca->wake_retrans_timer);
>         qca->hu = NULL;
>
>         kfree_skb(qca->rx_skb);
>

Acked-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

-- 
Luiz Augusto von Dentz