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

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Date:   Wed, 27 Apr 2022 14:47:12 +0200
From:   Andreas Larsson <andreas@...sler.com>
To:     Oliver Hartkopp <socketcan@...tkopp.net>,
        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 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>

-- 
Andreas Larsson