Re: [PATCH 02/11] drivers: usb: host: Fix deadlock in oxu_bus_suspend()

[Oliver Neukum <oneukum@suse.com>]

On 07.04.22 08:33, Duoming Zhou wrote:
> There is a deadlock in oxu_bus_suspend(), which is shown below:
>
>    (Thread 1)              |      (Thread 2)
>                            | timer_action()
> oxu_bus_suspend()          |  mod_timer()
>  spin_lock_irq() //(1)     |  (wait a time)
>  ...                       | oxu_watchdog()
>  del_timer_sync()          |  spin_lock_irq() //(2)
>  (wait timer to stop)      |  ...
>
> We hold oxu->lock in position (1) of thread 1, and use
> del_timer_sync() to wait timer to stop, but timer handler
> also need oxu->lock in position (2) of thread 2. As a result,
> oxu_bus_suspend() will block forever.
>
> This patch extracts del_timer_sync() from the protection of
> spin_lock_irq(), which could let timer handler to obtain
> the needed lock.
Good catch.
> Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
> ---
>  drivers/usb/host/oxu210hp-hcd.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/usb/host/oxu210hp-hcd.c b/drivers/usb/host/oxu210hp-hcd.c
> index b741670525e..ee403df3309 100644
> --- a/drivers/usb/host/oxu210hp-hcd.c
> +++ b/drivers/usb/host/oxu210hp-hcd.c
> @@ -3909,8 +3909,10 @@ static int oxu_bus_suspend(struct usb_hcd *hcd)
>  		}
>  	}
>  
> +	spin_unlock_irq(&oxu->lock);
>  	/* turn off now-idle HC */
>  	del_timer_sync(&oxu->watchdog);
> +	spin_lock_irq(&oxu->lock);
>  	ehci_halt(oxu);
>  	hcd->state = HC_STATE_SUSPENDED;
>  

What is the lock protecting at that stage? Why not just drop it earlier.

    Regards
        Oliver