2022-04-07 15:47:04

by Oliver Neukum

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 <[email protected]>
> ---
> 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

2022-04-07 20:52:38

by Duoming Zhou

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

Hello,

On Thu, 7 Apr 2022 10:01:43 +0200 Oliver Neukum wrote:

> 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 <[email protected]>
> > ---
> > 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.

I think there is a race condition between oxu_bus_suspend() and oxu_stop(),
so I think we could not drop the oxu->lock earlier.

(Thread 1) | (Thread 2)
oxu_bus_suspend() | oxu_stop()
|
hcd->state = HC_STATE_SUSPENDED; | spin_lock_irq(&oxu->lock);
... |
writel(mask, &oxu->regs->intr_enable); | ...
| writel(0, &oxu->regs->intr_enable);
readl(&oxu->regs->intr_enable); |

The oxu->regs->intr_enable is set to 0 in oxu_stop(), and the readl() in
oxu_bus_suspend() will read the wrong value.

Thanks a lot for your time and advice. If you have questions, welcome to ask me.

Best regards,
Duoming Zhou