Re: [PATCH] rcu: update: Check rcu_bh_lock_map state in rcu_read_lock_bh_held

[Neeraj Upadhyay]

On 6/22/2021 11:28 PM, Paul E. McKenney wrote:
> On Tue, Jun 22, 2021 at 05:35:21PM +0530, Neeraj Upadhyay wrote:
> > In addition to irq and softirq state, check rcu_bh_lock_map
> > state, to decide whether RCU bh lock is held.
> >
> > Signed-off-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
>
> My initial reaction was that "in_softirq() || irqs_disabled()" covers
> it because rcu_read_lock_bh() disables BH.  But you are right that it
> does seem a bit silly to ignore lockdep.
>
> So would it also make sense to have a WARN_ON_ONCE() if lockdep claims
> we are under rcu_read_lock_bh() protection, but "in_softirq() ||
> irqs_disabled()" think otherwise?
>
> 							Thanx, Paul

After thinking more on this, looks like one intention of not
having lockdep check here was to catch scenarios where some code enables 
bh after doing rcu_read_lock_bh(), as is mentioned in the comment above 
rcu_read_lock_bh_held():

  Note that if someone uses
  rcu_read_lock_bh(), but then later enables BH, lockdep (if enabled)
  will show the situation.  This is useful for debug checks in functions
  that require that they be called within an RCU read-side critical
  section.

Client users seem to be doing lockdep checks on returned value:
drivers/net/wireguard/peer.c
   RCU_LOCKDEP_WARN(!rcu_read_lock_bh_held(),

Similarly, any rcu_dereference_check(..., rcu_read_lock_bh_held()) usage 
also triggers warning, if bh is enabled, inside rcu_read_lock_bh()
section.

So, using 'in_softirq() || irqs_disabled()' condition looks to be 
sufficient condition, to mark all read lock bh regions and adding '|| 
lock_is_held(&rcu_bh_lock_map)' to this condition does not seem to fit
well with the RCU_LOCKDEP_WARN(!rcu_read_lock_bh_held()) and 
rcu_dereference_check(..., rcu_read_lock_bh_held()) calls, if we hit
the scenario, where bh lockmap state (shows bh lock acquired) conflicts 
with the softirq/irq state .



Thanks
Neeraj

> > ---
> >   kernel/rcu/update.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> > index c21b38c..d416f1c 100644
> > --- a/kernel/rcu/update.c
> > +++ b/kernel/rcu/update.c
> > @@ -333,7 +333,7 @@ int rcu_read_lock_bh_held(void)
> >   
> >   	if (rcu_read_lock_held_common(&ret))
> >   		return ret;
> > -	return in_softirq() || irqs_disabled();
> > +	return lock_is_held(&rcu_bh_lock_map) || in_softirq() || irqs_disabled();
> >   }
> >   EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);
> >   
> > --
> > QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> > hosted by The Linux Foundation

--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a 
member of the Code Aurora Forum, hosted by The Linux Foundation