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

[Zhihao Cheng]

在 2024/7/12 22:37, Theodore Ts'o 写道:
>> 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():

The counter of ea_inode could become zero before the file inode, 
according to the following process:
path_getxattr
  user_path_at(&path) // get file dentry and file inode
  getxattr
   ext4_xattr_get
    ext4_xattr_ibody_get
     ext4_xattr_inode_get
      ext4_xattr_inode_iget(&ea_inode)  // ea_inode->i_count = 1
      iput(ea_inode) // ea_inode->i_count = 0, put it into lru
  path_put(&path); // put file dentry and file 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?

The reproducer is in the second half of the ariticle, along with some of 
the solutions we tried.

Reproducer:
===========

https://bugzilla.kernel.org/show_bug.cgi?id=219022

About solutions
===============

[...]