Re: [PATCH] mm/swap: fix swap_info_struct race between swapoff and get_swap_pages()

[Matthew Wilcox <willy@infradead.org>]

On Sun, Apr 02, 2023 at 06:19:20AM +0800, Rongwei Wang wrote:
> Without this modification, a core will wait (mostly)
> 'swap_info_struct->lock' when completing
> 'del_from_avail_list(p)'. Immediately, other cores
> soon calling 'add_to_avail_list()' to add the same
> object again when acquiring the lock that released
> by former. It's not the desired result but exists
> indeed. This case can be described as below:

This feels like a very verbose way of saying

"The si->lock must be held when deleting the si from the
available list.  Otherwise, another thread can re-add the
si to the available list, which can lead to memory corruption.
The only place we have found where this happens is in the
swapoff path."

> +++ b/mm/swapfile.c
> @@ -2610,8 +2610,12 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>  		spin_unlock(&swap_lock);
>  		goto out_dput;
>  	}
> -	del_from_avail_list(p);
> +	/*
> +	 * Here lock is used to protect deleting and SWP_WRITEOK clearing
> +	 * can be seen concurrently.
> +	 */

This comment isn't necessary.  But I would add a lockdep assert inside
__del_from_avail_list() that p->lock is held.

>  	spin_lock(&p->lock);
> +	del_from_avail_list(p);
>  	if (p->prio < 0) {
>  		struct swap_info_struct *si = p;
>  		int nid;
> -- 
> 2.27.0
> 
>