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

[Jan Kara]

On Fri 12-07-24 10:37:08, Theodore Ts'o wrote:
> 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?

No, it looks like a real problem and I agree with the analysis. We don't
hold ea_inode reference (i.e., ea_inode->i_count) from a normal inode. The
normal inode just owns that that special on-disk xattr reference. Standard
inode references are acquired and dropped as needed.

And this is exactly the problem: ext4_xattr_inode_dec_ref_all() called from
evict() needs to lookup the ea_inode and iget() it. So if we are processing
a list of inodes to dispose, all inodes have I_FREEING bit already set and
if ea_inode and its parent normal inode are both in the list, then the
evict()->ext4_xattr_inode_dec_ref_all()->iget() will deadlock.

Normally we don't hit this path because LRU list walk is not handling
inodes with 0 link count. But a race with unlink can make that happen with
iput() from inode_lru_isolate().

I'm pondering about the best way to fix this. Maybe we could handle the
need for inode pinning in inode_lru_isolate() in a similar way as in
writeback code so that last iput() cannot happen from inode_lru_isolate().
In writeback we use I_SYNC flag to pin the inode and evict() waits for this
flag to clear. I'll probably sleep to it and if I won't find it too
disgusting to live tomorrow, I can code it.

								Honza
-- 
Jan Kara <jack at suse.com>
SUSE Labs, CR