Subject: Re: [PATCH 09/11] drivers: infiniband: hw: Fix deadlock in irdma_cleanup_cm_core()

From: Dan Carpenter <[email-masked]>
Date: Thu, 7 Apr 2022 14:24:56 +0300

On Thu, Apr 07, 2022 at 02:37:12PM +0800, Duoming Zhou wrote:
> There is a deadlock in irdma_cleanup_cm_core(), which is shown
> below:
> 
>    (Thread 1)              |      (Thread 2)
>                            | irdma_schedule_cm_timer()
> irdma_cleanup_cm_core()    |  add_timer()
>  spin_lock_irqsave() //(1) |  (wait a time)
>  ...                       | irdma_cm_timer_tick()
>  del_timer_sync()          |  spin_lock_irqsave() //(2)
>  (wait timer to stop)      |  ...
> 
> We hold cm_core->ht_lock in position (1) of thread 1 and
> use del_timer_sync() to wait timer to stop, but timer handler
> also need cm_core->ht_lock in position (2) of thread 2.
> As a result, irdma_cleanup_cm_core() 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 <[email-masked]>
> ---
>  drivers/infiniband/hw/irdma/cm.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/infiniband/hw/irdma/cm.c b/drivers/infiniband/hw/irdma/cm.c
> index dedb3b7edd8..019dd8bfe08 100644
> --- a/drivers/infiniband/hw/irdma/cm.c
> +++ b/drivers/infiniband/hw/irdma/cm.c
> @@ -3252,8 +3252,11 @@ void irdma_cleanup_cm_core(struct irdma_cm_core *cm_core)
>  		return;
>  
>  	spin_lock_irqsave(&cm_core->ht_lock, flags);
> -	if (timer_pending(&cm_core->tcp_timer))
> +	if (timer_pending(&cm_core->tcp_timer)) {
> +		spin_unlock_irqrestore(&cm_core->ht_lock, flags);
>  		del_timer_sync(&cm_core->tcp_timer);
> +		spin_lock_irqsave(&cm_core->ht_lock, flags);
> +	}
>  	spin_unlock_irqrestore(&cm_core->ht_lock, flags);

This lock doesn't seem to be protecting anything.  Also do we need to
check timer_pending()?  I think the del_timer_sync() function will just
return directly if there isn't a pending lock?

regards,
dan carpenter