Re: [PATCH net] drivers: net: can: Fix deadlock in grcan_close()

[Oliver Hartkopp]

On 25.04.22 06:24, Duoming Zhou wrote:
> There are deadlocks caused by del_timer_sync(&priv->hang_timer)
> and del_timer_sync(&priv->rr_timer) in grcan_close(), one of
> the deadlocks are shown below:
>
>     (Thread 1)              |      (Thread 2)
>                             | grcan_reset_timer()
> grcan_close()              |  mod_timer()
>   spin_lock_irqsave() //(1) |  (wait a time)
>   ...                       | grcan_initiate_running_reset()
>   del_timer_sync()          |  spin_lock_irqsave() //(2)
>   (wait timer to stop)      |  ...
>
> We hold priv->lock in position (1) of thread 1 and use
> del_timer_sync() to wait timer to stop, but timer handler
> also need priv->lock in position (2) of thread 2.
> As a result, grcan_close() will block forever.
>
> This patch extracts del_timer_sync() from the protection of
> spin_lock_irqsave(), which could let timer handler to obtain
> the needed lock.
>
> Signed-off-by: Duoming Zhou <duoming@xxxxxxxxxx>
> ---
>   drivers/net/can/grcan.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/drivers/net/can/grcan.c b/drivers/net/can/grcan.c
> index d0c5a7a60da..1189057b5d6 100644
> --- a/drivers/net/can/grcan.c
> +++ b/drivers/net/can/grcan.c
> @@ -1102,8 +1102,10 @@ static int grcan_close(struct net_device *dev)
>   
>   	priv->closing = true;
>   	if (priv->need_txbug_workaround) {
> +		spin_unlock_irqrestore(&priv->lock, flags);
>   		del_timer_sync(&priv->hang_timer);
>   		del_timer_sync(&priv->rr_timer);
> +		spin_lock_irqsave(&priv->lock, flags);

It looks weird to unlock and re-lock the operations like this. This 
breaks the intended locking for the closing process.

Isn't there any possibility to e.g. move that entire if-section before 
the lock?

>   	}
>   	netif_stop_queue(dev);
>   	grcan_stop_hardware(dev);

Regards,
Oliver