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@xxxxxxxxxx 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@xxxxxxxxxx>
> > > > > ---
> > > > > 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
