netdev - Re: [PATCH 02/11] drivers: usb: host: Fix deadlock in oxu_bus

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Date:   Thu, 7 Apr 2022 10:01:43 +0200
From:   Oliver Neukum <oneukum@...e.com>
To:     Duoming Zhou <duoming@....edu.cn>, linux-kernel@...r.kernel.org
CC:     chris@...kel.net, jcmvbkbc@...il.com, mustafa.ismail@...el.com,
        shiraz.saleem@...el.com, jgg@...pe.ca, wg@...ndegger.com,
        mkl@...gutronix.de, davem@...emloft.net, kuba@...nel.org,
        pabeni@...hat.com, jes@...ined-monkey.org,
        gregkh@...uxfoundation.org, jirislaby@...nel.org,
        alexander.deucher@....com, linux-xtensa@...ux-xtensa.org,
        linux-rdma@...r.kernel.org, linux-can@...r.kernel.org,
        netdev@...r.kernel.org, linux-hippi@...site.dk,
        linux-staging@...ts.linux.dev, linux-serial@...r.kernel.org,
        linux-usb@...r.kernel.org
Subject: Re: [PATCH 02/11] drivers: usb: host: Fix deadlock in
 oxu_bus_suspend()



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@....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