linux-kernel - Re: [PATCH RT] locking: Make spinlock_t and rwlock

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Date:   Tue, 19 Nov 2019 06:47:10 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH RT] locking: Make spinlock_t and rwlock_t a RCU section
 on RT

On Tue, Nov 19, 2019 at 09:46:40AM +0100, Sebastian Andrzej Siewior wrote:
> On !RT a locked spinlock_t and rwlock_t disables preemption which
> implies a RCU read section. There is code that relies on that behaviour.
> 
> Add an explicit RCU read section on RT while a sleeping lock (a lock
> which would disables preemption on !RT) acquired.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Acked-by: Paul E. McKenney <paulmck@...nel.org>

> ---
>  kernel/locking/rtmutex.c   | 6 ++++++
>  kernel/locking/rwlock-rt.c | 6 ++++++
>  2 files changed, 12 insertions(+)
> 
> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> index cf09f8e7ed0f4..602eb7821a1b1 100644
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -1141,6 +1141,7 @@ void __sched rt_spin_lock_slowunlock(struct rt_mutex *lock)
>  void __lockfunc rt_spin_lock(spinlock_t *lock)
>  {
>  	sleeping_lock_inc();
> +	rcu_read_lock();
>  	migrate_disable();
>  	spin_acquire(&lock->dep_map, 0, 0, _RET_IP_);
>  	rt_spin_lock_fastlock(&lock->lock, rt_spin_lock_slowlock);
> @@ -1156,6 +1157,7 @@ void __lockfunc __rt_spin_lock(struct rt_mutex *lock)
>  void __lockfunc rt_spin_lock_nested(spinlock_t *lock, int subclass)
>  {
>  	sleeping_lock_inc();
> +	rcu_read_lock();
>  	migrate_disable();
>  	spin_acquire(&lock->dep_map, subclass, 0, _RET_IP_);
>  	rt_spin_lock_fastlock(&lock->lock, rt_spin_lock_slowlock);
> @@ -1169,6 +1171,7 @@ void __lockfunc rt_spin_unlock(spinlock_t *lock)
>  	spin_release(&lock->dep_map, 1, _RET_IP_);
>  	rt_spin_lock_fastunlock(&lock->lock, rt_spin_lock_slowunlock);
>  	migrate_enable();
> +	rcu_read_unlock();
>  	sleeping_lock_dec();
>  }
>  EXPORT_SYMBOL(rt_spin_unlock);
> @@ -1200,6 +1203,7 @@ int __lockfunc rt_spin_trylock(spinlock_t *lock)
>  	ret = __rt_mutex_trylock(&lock->lock);
>  	if (ret) {
>  		spin_acquire(&lock->dep_map, 0, 1, _RET_IP_);
> +		rcu_read_lock();
>  	} else {
>  		migrate_enable();
>  		sleeping_lock_dec();
> @@ -1216,6 +1220,7 @@ int __lockfunc rt_spin_trylock_bh(spinlock_t *lock)
>  	ret = __rt_mutex_trylock(&lock->lock);
>  	if (ret) {
>  		sleeping_lock_inc();
> +		rcu_read_lock();
>  		migrate_disable();
>  		spin_acquire(&lock->dep_map, 0, 1, _RET_IP_);
>  	} else
> @@ -1232,6 +1237,7 @@ int __lockfunc rt_spin_trylock_irqsave(spinlock_t *lock, unsigned long *flags)
>  	ret = __rt_mutex_trylock(&lock->lock);
>  	if (ret) {
>  		sleeping_lock_inc();
> +		rcu_read_lock();
>  		migrate_disable();
>  		spin_acquire(&lock->dep_map, 0, 1, _RET_IP_);
>  	}
> diff --git a/kernel/locking/rwlock-rt.c b/kernel/locking/rwlock-rt.c
> index c3b91205161cc..0ae8c62ea8320 100644
> --- a/kernel/locking/rwlock-rt.c
> +++ b/kernel/locking/rwlock-rt.c
> @@ -310,6 +310,7 @@ int __lockfunc rt_read_trylock(rwlock_t *rwlock)
>  	ret = do_read_rt_trylock(rwlock);
>  	if (ret) {
>  		rwlock_acquire_read(&rwlock->dep_map, 0, 1, _RET_IP_);
> +		rcu_read_lock();
>  	} else {
>  		migrate_enable();
>  		sleeping_lock_dec();
> @@ -327,6 +328,7 @@ int __lockfunc rt_write_trylock(rwlock_t *rwlock)
>  	ret = do_write_rt_trylock(rwlock);
>  	if (ret) {
>  		rwlock_acquire(&rwlock->dep_map, 0, 1, _RET_IP_);
> +		rcu_read_lock();
>  	} else {
>  		migrate_enable();
>  		sleeping_lock_dec();
> @@ -338,6 +340,7 @@ EXPORT_SYMBOL(rt_write_trylock);
>  void __lockfunc rt_read_lock(rwlock_t *rwlock)
>  {
>  	sleeping_lock_inc();
> +	rcu_read_lock();
>  	migrate_disable();
>  	rwlock_acquire_read(&rwlock->dep_map, 0, 0, _RET_IP_);
>  	do_read_rt_lock(rwlock);
> @@ -347,6 +350,7 @@ EXPORT_SYMBOL(rt_read_lock);
>  void __lockfunc rt_write_lock(rwlock_t *rwlock)
>  {
>  	sleeping_lock_inc();
> +	rcu_read_lock();
>  	migrate_disable();
>  	rwlock_acquire(&rwlock->dep_map, 0, 0, _RET_IP_);
>  	do_write_rt_lock(rwlock);
> @@ -358,6 +362,7 @@ void __lockfunc rt_read_unlock(rwlock_t *rwlock)
>  	rwlock_release(&rwlock->dep_map, 1, _RET_IP_);
>  	do_read_rt_unlock(rwlock);
>  	migrate_enable();
> +	rcu_read_unlock();
>  	sleeping_lock_dec();
>  }
>  EXPORT_SYMBOL(rt_read_unlock);
> @@ -367,6 +372,7 @@ void __lockfunc rt_write_unlock(rwlock_t *rwlock)
>  	rwlock_release(&rwlock->dep_map, 1, _RET_IP_);
>  	do_write_rt_unlock(rwlock);
>  	migrate_enable();
> +	rcu_read_unlock();
>  	sleeping_lock_dec();
>  }
>  EXPORT_SYMBOL(rt_write_unlock);
> -- 
> 2.24.0
>