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

[duoming@zju.edu.cn]

Hello,

On Thu, 7 Apr 2022 17:36:12 +0000 Saleem, Shiraz wrote:
 
> > Subject: Re: Re: [PATCH 09/11] drivers: infiniband: hw: Fix deadlock in
> > irdma_cleanup_cm_core()
> > 
> > On Thu, Apr 07, 2022 at 08:54:13PM +0800, duoming@zju.edu.cn 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 <duoming@zju.edu.cn>
> > > > > ---
> > > > >  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.
> > 
> Agreed on this fix.
> 
> We should not lock around del_timer_sync or need to check on timer_pending.
> 
> However, we do need serialize addition of a timer which can be called from multiple paths, i.e. the timer handler and irdma_schedule_cm_timer.

I think we should replace the check "if (!timer_pending(&cm_core->tcp_timer))" to
"if (timer_pending(&cm_core->tcp_timer))" in irdma_cm_timer_tick(), and replace
"if (!was_timer_set)" to "if (was_timer_set)" in irdma_schedule_cm_timer() in order
to guarantee the timer could be executed. I will send the modified patch as soon as 
possible.

Best regards,
Duoming Zhou