Re: [PATCH] mm/vmalloc: Fix unlock order in s_stop()

[Uladzislau Rezki <urezki@gmail.com>]

On Sun, Dec 13, 2020 at 01:08:43PM -0500, Waiman Long wrote:
> When multiple locks are acquired, they should be released in reverse
> order. For s_start() and s_stop() in mm/vmalloc.c, that is not the
> case.
> 
>   s_start: mutex_lock(&vmap_purge_lock); spin_lock(&vmap_area_lock);
>   s_stop : mutex_unlock(&vmap_purge_lock); spin_unlock(&vmap_area_lock);
> 
> This unlock sequence, though allowed, is not optimal. If a waiter is
> present, mutex_unlock() will need to go through the slowpath of waking
> up the waiter with preemption disabled. Fix that by releasing the
> spinlock first before the mutex.
> 
> Fixes: e36176be1c39 ("mm/vmalloc: rework vmap_area_lock")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  mm/vmalloc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 6ae491a8b210..75913f685c71 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -3448,11 +3448,11 @@ static void *s_next(struct seq_file *m, void *p, loff_t *pos)
>  }
>  
>  static void s_stop(struct seq_file *m, void *p)
> -	__releases(&vmap_purge_lock)
>  	__releases(&vmap_area_lock)
> +	__releases(&vmap_purge_lock)
>  {
> -	mutex_unlock(&vmap_purge_lock);
>  	spin_unlock(&vmap_area_lock);
> +	mutex_unlock(&vmap_purge_lock);
>  }
>  
>  static void show_numa_info(struct seq_file *m, struct vm_struct *v)
BTW, if navigation over both list is an issue, for example when there
are multiple heavy readers of /proc/vmallocinfo, i think, it make sense
to implement RCU safe lists iteration and get rid of both locks.

As for the patch: Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

Thanks!

--
Vlad Rezki