Re: [BUG REPORT] potential deadlock in inode evicting under the inode lru traversing context on ext4 and ubifs

[Theodore Ts'o]

On Fri, Jul 12, 2024 at 02:27:20PM +0800, Zhihao Cheng wrote:
> Problem description
> ===================
> 
> The inode reclaiming process(See function prune_icache_sb) collects all
> reclaimable inodes and mark them with I_FREEING flag at first, at that
> time, other processes will be stuck if they try getting these inodes(See
> function find_inode_fast), then the reclaiming process destroy the
> inodes by function dispose_list().
> Some filesystems(eg. ext4 with ea_inode feature, ubifs with xattr) may
> do inode lookup in the inode evicting callback function, if the inode
> lookup is operated under the inode lru traversing context, deadlock
> problems may happen.
> 
> Case 1: In function ext4_evict_inode(), the ea inode lookup could happen
> if ea_inode feature is enabled, the lookup process will be stuck under
> the evicting context like this:
> 
>  1. File A has inode i_reg and an ea inode i_ea
>  2. getfattr(A, xattr_buf) // i_ea is added into lru // lru->i_ea
>  3. Then, following three processes running like this:
> 
>     PA                              PB
>  echo 2 > /proc/sys/vm/drop_caches
>   shrink_slab
>    prune_dcache_sb
>    // i_reg is added into lru, lru->i_ea->i_reg
>    prune_icache_sb
>     list_lru_walk_one
>      inode_lru_isolate
>       i_ea->i_state |= I_FREEING // set inode state
>       i_ea->i_state |= I_FREEING // set inode state

Um, I don't see how this can happen.  If the ea_inode is in use,
i_count will be greater than zero, and hence the inode will never be
go down the rest of the path in inode_lru_inode():

	if (atomic_read(&inode->i_count) ||
	    ...) {
		list_lru_isolate(lru, &inode->i_lru);
		spin_unlock(&inode->i_lock);
		this_cpu_dec(nr_unused);
		return LRU_REMOVED;
	}

Do you have an actual reproduer which triggers this?  Or would this
happen be any chance something that was dreamed up with DEPT?

       	      	     	       	    		- Ted