On Tue, Jun 18, 2019 at 08:19:05PM -0500, Scott Wood wrote:
> A plain local_bh_disable() is documented as creating an RCU critical
> section, and (at least) rcutorture expects this to be the case. However,
> in_softirq() doesn't block a grace period on PREEMPT_RT, since RCU checks
> preempt_count() directly. Even if RCU were changed to check
> in_softirq(), that wouldn't allow blocked BH disablers to be boosted.
>
> Fix this by calling rcu_read_lock() from local_bh_disable(), and update
> rcu_read_lock_bh_held() accordingly.
>
> Signed-off-by: Scott Wood <swood@xxxxxxxxxx>
> ---
> include/linux/rcupdate.h | 4 ++++
> kernel/rcu/update.c | 4 ++++
> kernel/softirq.c | 12 +++++++++---
> 3 files changed, 17 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index fb267bc04fdf..aca4e5e25ace 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -637,10 +637,12 @@ static inline void rcu_read_unlock(void)
> static inline void rcu_read_lock_bh(void)
> {
> local_bh_disable();
> +#ifndef CONFIG_PREEMPT_RT_FULL
How about this instead?
if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL))
return;
And similarly below.
> __acquire(RCU_BH);
> rcu_lock_acquire(&rcu_bh_lock_map);
> RCU_LOCKDEP_WARN(!rcu_is_watching(),
> "rcu_read_lock_bh() used illegally while idle");
> +#endif
> }
>
> /*
> @@ -650,10 +652,12 @@ static inline void rcu_read_lock_bh(void)
> */
> static inline void rcu_read_unlock_bh(void)
> {
> +#ifndef CONFIG_PREEMPT_RT_FULL
> RCU_LOCKDEP_WARN(!rcu_is_watching(),
> "rcu_read_unlock_bh() used illegally while idle");
> rcu_lock_release(&rcu_bh_lock_map);
> __release(RCU_BH);
> +#endif
> local_bh_enable();
> }
>
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index 3700b730ea55..eb653a329e0b 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -307,7 +307,11 @@ int rcu_read_lock_bh_held(void)
> return 0;
> if (!rcu_lockdep_current_cpu_online())
> return 0;
> +#ifdef CONFIG_PREEMPT_RT_FULL
> + return lock_is_held(&rcu_lock_map) || irqs_disabled();
> +#else
> return in_softirq() || irqs_disabled();
> +#endif
> }
> EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);
>
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index 473369122ddd..eb46dd3ff92d 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -121,8 +121,10 @@ void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
> long soft_cnt;
>
> WARN_ON_ONCE(in_irq());
> - if (!in_atomic())
> + if (!in_atomic()) {
> local_lock(bh_lock);
> + rcu_read_lock();
> + }
> soft_cnt = this_cpu_inc_return(softirq_counter);
> WARN_ON_ONCE(soft_cnt == 0);
>
> @@ -155,8 +157,10 @@ void _local_bh_enable(void)
> local_irq_restore(flags);
> #endif
>
> - if (!in_atomic())
> + if (!in_atomic()) {
> + rcu_read_unlock();
> local_unlock(bh_lock);
> + }
> }
>
> void _local_bh_enable_rt(void)
> @@ -189,8 +193,10 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
> WARN_ON_ONCE(count < 0);
> local_irq_enable();
>
> - if (!in_atomic())
> + if (!in_atomic()) {
> + rcu_read_unlock();
> local_unlock(bh_lock);
> + }
>
> preempt_check_resched();
> }
And I have to ask...
What did you do to test this change to kernel/softirq.c? My past attempts
to do this sort of thing have always run afoul of open-coded BH transitions.
Thanx, Paul
