Messages in this thread Patch in this message | | | Subject | Re: [PATCH] rcu: update: Check rcu_bh_lock_map state in rcu_read_lock_bh_held | From | Neeraj Upadhyay <> | Date | Wed, 23 Jun 2021 00:38:09 +0530 |
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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@codeaurora.org> > > 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
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