Re: [PATCH] rcu: update: Check rcu_bh_lock_map state in rcu_read_lock_bh_held
From: Neeraj Upadhyay
Date: Tue Jun 22 2021 - 23:59:19 EST
On 6/23/2021 5:16 AM, Paul E. McKenney wrote:
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
I think this check is good to have inside rcu_read_lock_bh_held(), to
highlight this scenario explicitly; I am thinking, if it makes sense to
have lock_is_held(&rcu_bh_lock_map) check in rcu_softirq_qs() ?
Also, I think this check is more important for
rcu_read_lock_sched_held(), where lockdep state is used as a sufficient
condition, for marking a RCU
sched region. One more api is rcu_read_lock_any_held(), where we can
warn on conflicting cases.
int rcu_read_lock_sched_held(void)
{
bool ret;
if (rcu_read_lock_held_common(&ret))
return ret;
return lock_is_held(&rcu_sched_lock_map) || !preemptible();
}
Thanks
Neeraj
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);
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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