Re: [PATCH] adapt page_lock_anon_vma() to PREEMPT_RCU

[Andrew Morton <akpm@linux-foundation.org>]

> On Sun, 25 Feb 2007 23:06:21 +0300 Oleg Nesterov <oleg@tv-sign.ru> wrote:
> page_lock_anon_vma() uses spin_lock() to block RCU. This doesn't work with
> PREEMPT_RCU, we have to do rcu_read_lock() explicitely. Otherwise, it is
> theoretically possible that slab returns anon_vma's memory to the system
> before we do spin_unlock(&anon_vma->lock).
> 
> ...
>
> +static void page_unlock_anon_vma(struct anon_vma *anon_vma)
> +{
> +	spin_unlock(&anon_vma->lock);
> +	rcu_read_unlock();
>  }

It's a bit sad doing a double preempt_disable() for non-PREEMPT_RCU builds.

Perhaps we would benefit from a new rcu_read_lock_preempt_rcu() which is a
no-op if !PREEMPT_RCU.