linux-kernel - Re: RE: [PATCH V5] drivers: infiniband: hw: Fix deadlock in irdma_cleanup_cm

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Date:   Mon, 18 Apr 2022 23:41:00 +0800 (GMT+08:00)
From:   duoming@....edu.cn
To:     "Saleem, Shiraz" <shiraz.saleem@...el.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Ismail, Mustafa" <mustafa.ismail@...el.com>,
        "jgg@...pe.ca" <jgg@...pe.ca>, "leon@...nel.org" <leon@...nel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>
Subject: Re: RE: [PATCH V5] drivers: infiniband: hw: Fix deadlock in
 irdma_cleanup_cm_core()

Hello,

On Mon, 18 Apr 2022 14:57:06 +0000 Saleem, Shiraz 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 removes the check of timer_pending() in irdma_cleanup_cm_core(),
> > because the del_timer_sync() function will just return directly if there isn't a
> > pending timer. As a result, the lock is redundant, because there is no resource it
> > could protect.
> > 
> > Signed-off-by: Duoming Zhou <duoming@....edu.cn>
> > ---
> > Changes in V5:
> >   - Remove mod_timer() in irdma_schedule_cm_timer and irdma_cm_timer_tick.
> > 
> >  drivers/infiniband/hw/irdma/cm.c | 5 +----
> >  1 file changed, 1 insertion(+), 4 deletions(-)
> > 
> > diff --git a/drivers/infiniband/hw/irdma/cm.c b/drivers/infiniband/hw/irdma/cm.c
> > index dedb3b7edd8..4b6b1065f85 100644
> > --- a/drivers/infiniband/hw/irdma/cm.c
> > +++ b/drivers/infiniband/hw/irdma/cm.c
> > @@ -3251,10 +3251,7 @@ void irdma_cleanup_cm_core(struct
> > irdma_cm_core *cm_core)
> >  	if (!cm_core)
> >  		return;
> > 
> > -	spin_lock_irqsave(&cm_core->ht_lock, flags);
> > -	if (timer_pending(&cm_core->tcp_timer))
> > -		del_timer_sync(&cm_core->tcp_timer);
> > -	spin_unlock_irqrestore(&cm_core->ht_lock, flags);
> > +	del_timer_sync(&cm_core->tcp_timer);
> > 
> >  	destroy_workqueue(cm_core->event_wq);
> >  	cm_core->dev->ws_reset(&cm_core->iwdev->vsi);
> > --
> 
> I am not sure the deadlock is possible practically since all CM nodes should be culled by the time we get to irdma_cleanup_cm_core.

I think the deadlock is possible, because the timer is a delay mechanism that could execute at any time, although all CM nodes are culled.

> However, timer_pending check and locks are redundant and should be removed.
> 
> The subject line for patches to our driver are typically prefixed with "RDMA/irdma: "

I sent "[PATCH V6] RDMA/irdma: Fix deadlock in irdma_cleanup_cm_core()" just now.

Best regards,
Duoming Zhou