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

From: Dan Carpenter <[email-masked]>
Date: Thu, 7 Apr 2022 18:24:08 +0300

On Thu, Apr 07, 2022 at 08:54:13PM +0800, [email-masked] wrote:
> Hello,
> 
> On Thu, 7 Apr 2022 14:24:56 +0300 Dan Carpenter 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?
> 
> Thanks a lot for your advice, I will remove the timer_pending() and the
> redundant lock.

I didn't give any advice. :P I only ask questions when I don't know the
answers.  Someone probably needs to look at &cm_core->ht_lock and figure
out what it's protecting.

regards,
dan carpenter