linux-kernel - Re: [PATCH net] drivers: net: can: Fix deadlock in grcan

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Date:   Wed, 27 Apr 2022 18:25:40 +0200
From:   Oliver Hartkopp <socketcan@...tkopp.net>
To:     Andreas Larsson <andreas@...sler.com>,
        Duoming Zhou <duoming@....edu.cn>, linux-kernel@...r.kernel.org
Cc:     wg@...ndegger.com, mkl@...gutronix.de, davem@...emloft.net,
        kuba@...nel.org, pabeni@...hat.com, linux-can@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH net] drivers: net: can: Fix deadlock in grcan_close()



On 27.04.22 14:47, Andreas Larsson wrote:
> On 2022-04-26 21:12, Oliver Hartkopp wrote:
>> 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@....edu.cn>
>>> ---
>>>   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?
> 
> All functions wishing to start the timers both check priv->closing and
> then, if false, start the timer within the priv->lock spinlock. Given
> that, it should be ok that del_timer_sync is not done within the
> spinlock as therefore no one can restart any timers after priv->closing
> has been set to true.
> 
> It looks a bit weird, but setting priv->closing to true needs to happen
> within the priv->lock spinlock protection and needs to happen before
> del_timer_sync to avoid a race between grcan_close and someone starting
> the timer.
> 
> Reviewed-by: Andreas Larsson <andreas@...sler.com>
> 

Thanks Andreas!

Best regards,
Oliver