Re: [PATCH RT v3 1/5] rcu: Acquire RCU lock when disabling BHs

Joel Fernandes
2019-09-12T22:09:46Z

On Wed, Sep 11, 2019 at 05:57:25PM +0100, 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@redhat.com>

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

thanks,

- Joel

> ---
> v3: Remove change to rcu_read_lock_bh_held(), and move debug portions
> of rcu_read_[un]lock_bh() to separate functions
> ---
> include/linux/rcupdate.h | 40 ++++++++++++++++++++++++++++++++--------
> kernel/softirq.c | 12 +++++++++---
> 2 files changed, 41 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 388ace315f32..9ce7c5006a5e 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -600,6 +600,36 @@ static inline void rcu_read_unlock(void)
> rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
> }
>
> +#ifdef CONFIG_PREEMPT_RT_FULL
> +/*
> + * On RT, local_bh_disable() calls rcu_read_lock() -- no need to
> + * track it twice.
> + */
> +static inline void rcu_bh_lock_acquire(void)
> +{
> +}
> +
> +static inline void rcu_bh_lock_release(void)
> +{
> +}
> +#else
> +static inline void rcu_bh_lock_acquire(void)
> +{
> + __acquire(RCU_BH);
> + rcu_lock_acquire(&rcu_bh_lock_map);
> + RCU_LOCKDEP_WARN(!rcu_is_watching(),
> + "rcu_read_lock_bh() used illegally while idle");
> +}
> +
> +static inline void rcu_bh_lock_release(void)
> +{
> + 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
> +
> /**
> * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
> *
> @@ -615,10 +645,7 @@ static inline void rcu_read_unlock(void)
> static inline void rcu_read_lock_bh(void)
> {
> local_bh_disable();
> - __acquire(RCU_BH);
> - rcu_lock_acquire(&rcu_bh_lock_map);
> - RCU_LOCKDEP_WARN(!rcu_is_watching(),
> - "rcu_read_lock_bh() used illegally while idle");
> + rcu_bh_lock_acquire();
> }
>
> /*
> @@ -628,10 +655,7 @@ static inline void rcu_read_lock_bh(void)
> */
> static inline void rcu_read_unlock_bh(void)
> {
> - RCU_LOCKDEP_WARN(!rcu_is_watching(),
> - "rcu_read_unlock_bh() used illegally while idle");
> - rcu_lock_release(&rcu_bh_lock_map);
> - __release(RCU_BH);
> + rcu_bh_lock_release();
> local_bh_enable();
> }
>
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index d16d080a74f7..6080c9328df1 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -115,8 +115,10 @@ 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);
> current->softirq_count += SOFTIRQ_DISABLE_OFFSET;
> @@ -151,8 +153,10 @@ void _local_bh_enable(void)
> #endif
>
> current->softirq_count -= SOFTIRQ_DISABLE_OFFSET;
> - if (!in_atomic())
> + if (!in_atomic()) {
> + rcu_read_unlock();
> local_unlock(bh_lock);
> + }
> }
>
> void _local_bh_enable_rt(void)
> @@ -185,8 +189,10 @@ 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);
> + }
>
> current->softirq_count -= SOFTIRQ_DISABLE_OFFSET;
> preempt_check_resched();
> --
> 1.8.3.1
>