On Wed, Jun 23, 2021 at 12:38:09AM +0530, Neeraj Upadhyay wrote: > > > 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? > > 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 . That makes sense to me! But should there be checks somewhere for something like "lock_is_held(&rcu_bh_lock_map) && !in_softirq() && !irqs_disabled()"? Thanx, Paul > 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