linux-kernel - Re: [PATCH v2 7/9] locking/rwlocks: introduce write_lock

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Date:   Fri, 19 Nov 2021 11:35:16 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 7/9] locking/rwlocks: introduce write_lock_nested

On 2021-11-15 10:59:07 [-0800], Minchan Kim wrote:
> diff --git a/include/linux/rwlock_rt.h b/include/linux/rwlock_rt.h
> index 49c1f3842ed5..efd6da62c893 100644
> --- a/include/linux/rwlock_rt.h
> +++ b/include/linux/rwlock_rt.h
> @@ -28,6 +28,7 @@ extern void rt_read_lock(rwlock_t *rwlock);
>  extern int rt_read_trylock(rwlock_t *rwlock);
>  extern void rt_read_unlock(rwlock_t *rwlock);
>  extern void rt_write_lock(rwlock_t *rwlock);
> +extern void rt_write_lock_nested(rwlock_t *rwlock, int subclass);
>  extern int rt_write_trylock(rwlock_t *rwlock);
>  extern void rt_write_unlock(rwlock_t *rwlock);
>
> @@ -83,6 +84,11 @@ static __always_inline void write_lock(rwlock_t *rwlock)
>  	rt_write_lock(rwlock);
>  }
>  
> +static __always_inline void write_lock_nested(rwlock_t *rwlock, int subclass)
> +{
> +	rt_write_lock_nested(rwlock, subclass);
> +}
> +

These two hunks as-is don't work. You need a CONFIG_DEBUG_LOCK_ALLOC block and
in the !CONFIG_DEBUG_LOCK_ALLOC case you need

#define rt_write_lock_nested(lock, subclass)     rt_write_lock(((void)(subclass), (lock)))

>  static __always_inline void write_lock_bh(rwlock_t *rwlock)
>  {
>  	local_bh_disable();
> diff --git a/kernel/locking/spinlock_rt.c b/kernel/locking/spinlock_rt.c
> index b2e553f9255b..b82d346f1e00 100644
> --- a/kernel/locking/spinlock_rt.c
> +++ b/kernel/locking/spinlock_rt.c
> @@ -239,6 +239,18 @@ void __sched rt_write_lock(rwlock_t *rwlock)
>  }
>  EXPORT_SYMBOL(rt_write_lock);
>  
> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> +void __sched rt_write_lock_nested(rwlock_t *rwlock, int subclass)
> +{
> +	___might_sleep(__FILE__, __LINE__, 0);

This _must_ be rtlock_might_resched() like it is done in rt_write_lock()
above.

> +	rwlock_acquire(&rwlock->dep_map, subclass, 0, _RET_IP_);
> +	rwbase_write_lock(&rwlock->rwbase, TASK_RTLOCK_WAIT);
> +	rcu_read_lock();
> +	migrate_disable();
> +}
> +EXPORT_SYMBOL(rt_write_lock_nested);
> +#endif
> +
>  void __sched rt_read_unlock(rwlock_t *rwlock)
>  {
>  	rwlock_release(&rwlock->dep_map, _RET_IP_);

Sebastian