Re: [PATCH v6] ext4: fix ABBA deadlock in ext4_xattr_inode_cache_find()

["Colin King (gmail)" <colin.i.king@gmail.com>]

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Hi,

I gave this a 20 minute stress test with stress-ng on a 24 thread system 
and it now works perfectly. Thanks for the fix.

Tested-by: Colin Ian King <colin.i.king@gmail.com>


On 26/06/2026 06:48, Aditya Srivastava wrote:
> From: Aditya Prakash Srivastava <aditya.ansh182@gmail.com>
> 
> Syzbot/stress-ng reported an ABBA deadlock in ext4 when exercising
> concurrent xattr workloads (using the ea_inode mount/format option).
> 
> The deadlock occurs between the running transaction and the eviction
> thread:
> - Task 1 (stress-ng): Holds a reference to a shared mbcache_entry (ce)
>    and calls ext4_xattr_inode_cache_find() -> ext4_iget() to retrieve
>    the corresponding EA inode. Since the EA inode is currently being
>    evicted, ext4_iget() blocks in __wait_on_freeing_inode() waiting for
>    eviction to complete.
> - Task 2 (eviction thread): Currently evicting the same EA inode in
>    ext4_evict_ea_inode(). It calls mb_cache_entry_wait_unused(oe) which
>    blocks waiting for Task 1 to release the reference to the mbcache_entry.
> 
> To break this deadlock, implement a new ext4_iget() configuration flag
> named EXT4_IGET_NOWAIT. When set, perform a non-blocking lookup of the
> inode via VFS's find_inode_nowait() API.
> 
> If the inode is currently being evicted (marked with I_FREEING or
> I_WILL_FREE) or created (I_CREATING), or if it is not present in the VFS
> inode cache (cache miss), simply skip it (returning -ENOENT) rather than
> waiting for eviction/creation to complete, breaking the ABBA cycle.
> 
> Since we return -ENOENT immediately on a cache miss, we never attempt to
> allocate a new inode or call iget_locked(), completely eliminating any
> TOCTOU race window.
> 
> If the returned inode is I_NEW, wait for its initialization to clear via
> wait_on_new_inode(). If initialization fails and the inode is unhashed
> during wait_on_new_inode() waking up (e.g., due to an I/O read error in
> another thread), safely drop the reference and return -ENOENT. This
> unhashed check is executed unconditionally on all cache-hit pathways to
> properly handle concurrent initialization failures.
> 
> Finally, standard validation checks (including is_bad_inode,
> EXT4_EA_INODE_FL, file_acl, and xattr flags) are executed as normal inside
> check_igot_inode() to fully guarantee VFS-layer safety.
> 
> In ext4_xattr_inode_cache_find(), invoke ext4_iget() with the new
> EXT4_IGET_NOWAIT flag to perform the non-blocking cache search.
> 
> Suggested-by: Jan Kara <jack@suse.cz>
> Reported-by: Colin Ian King <colin.i.king@gmail.com>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219283
> Fixes: 0a46ef234756 ("ext4: do not create EA inode under buffer lock")
> Signed-off-by: Aditya Prakash Srivastava <aditya.ansh182@gmail.com>
> ---
> Changes in v6:
>    - Drop the redundant is_freeing tracking variable completely, as pointed
>      out by Jan Kara.
>    - Return -ENOENT instead of -ESTALE on cache misses and unhashed inode
>      checks, as requested by Jan Kara.
> 
> Changes in v5:
>    - Address two critical concurrency issues flagged by the Sashiko AI bot in v4:
>      1. Resolve the Time-Of-Check to Time-Of-Use (TOCTOU) race window between
>         find_inode_nowait() and iget_locked() by returning -ESTALE immediately
>         on a VFS cache miss. This completely bypasses fallback to iget_locked()
>         and prevents potential ABBA deadlocks.
>      2. Fix the improperly nested inode_unhashed() safety check by moving it
>         outside the I_NEW condition block, ensuring it runs unconditionally
>         on all cache-hit pathways to prevent false-positive filesystem
>         corruption errors during concurrent initialization failures.
> 
> Changes in v4:
>    - Check if the inode was unhashed during wait_on_new_inode() waking up
>      to handle transient initialization failures (like I/O read errors)
>      gracefully. Dropping the reference and returning -ESTALE prevents
>      false filesystem corruption errors (__ext4_error), as found by the
>      Sashiko AI bot.
> 
> Changes in v3:
>    - Implement a new ext4_iget() configuration flag named EXT4_IGET_NOWAIT to
>      fully contain the non-blocking lookup and VFS-level validations within
>      inode.c, as requested by Jan Kara.
>    - Skip inodes currently being created (I_CREATING), following Jan Kara's
>      direct feedback.
>    - Remove all open-coded match helpers and VFS state-checks from xattr.c.
> 
> Changes in v2:
>    - Read inode state locklessly using inode_state_read_once() to resolve
>      a lockdep assertion on cache hit.
>    - Manually restore essential inode/ea_inode validations on the retrieved
>      inode (is_bad_inode, EXT4_EA_INODE_FL, file_acl, and xattr checks) to
>      match VFS safety guarantees and prevent using corrupted/failed inodes.
> 
>   fs/ext4/ext4.h  |  3 ++-
>   fs/ext4/inode.c | 35 ++++++++++++++++++++++++++++++++---
>   fs/ext4/xattr.c |  2 +-
>   3 files changed, 35 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index b37c136ea3ab..c76dd0bdd3d8 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -3144,7 +3144,8 @@ typedef enum {
>   	EXT4_IGET_SPECIAL =	0x0001, /* OK to iget a system inode */
>   	EXT4_IGET_HANDLE = 	0x0002,	/* Inode # is from a handle */
>   	EXT4_IGET_BAD =		0x0004, /* Allow to iget a bad inode */
> -	EXT4_IGET_EA_INODE =	0x0008	/* Inode should contain an EA value */
> +	EXT4_IGET_EA_INODE =	0x0008,	/* Inode should contain an EA value */
> +	EXT4_IGET_NOWAIT =	0x0010	/* Non-blocking lookup (skip if freeing) */
>   } ext4_iget_flags;
>   
>   extern struct inode *__ext4_iget(struct super_block *sb, unsigned long ino,
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index ce99807c5f5b..a091a43959a3 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -5270,6 +5270,20 @@ void ext4_set_inode_mapping_order(struct inode *inode)
>   	mapping_set_folio_order_range(inode->i_mapping, min_order, max_order);
>   }
>   
> +static int ext4_iget_match(struct inode *inode, u64 ino, void *data)
> +{
> +	if (inode->i_ino != ino)
> +		return 0;
> +	spin_lock(&inode->i_lock);
> +	if (inode_state_read(inode) & (I_FREEING | I_WILL_FREE | I_CREATING)) {
> +		spin_unlock(&inode->i_lock);
> +		return -1;
> +	}
> +	__iget(inode);
> +	spin_unlock(&inode->i_lock);
> +	return 1;
> +}
> +
>   struct inode *__ext4_iget(struct super_block *sb, unsigned long ino,
>   			  ext4_iget_flags flags, const char *function,
>   			  unsigned int line)
> @@ -5298,9 +5312,24 @@ struct inode *__ext4_iget(struct super_block *sb, unsigned long ino,
>   		return ERR_PTR(-EFSCORRUPTED);
>   	}
>   
> -	inode = iget_locked(sb, ino);
> -	if (!inode)
> -		return ERR_PTR(-ENOMEM);
> +	if (flags & EXT4_IGET_NOWAIT) {
> +		inode = find_inode_nowait(sb, ino, ext4_iget_match, NULL);
> +		if (!inode)
> +			return ERR_PTR(-ENOENT);
> +
> +		if (inode_state_read_once(inode) & I_NEW)
> +			wait_on_new_inode(inode);
> +
> +		if (unlikely(inode_unhashed(inode))) {
> +			iput(inode);
> +			return ERR_PTR(-ENOENT);
> +		}
> +	} else {
> +		inode = iget_locked(sb, ino);
> +		if (!inode)
> +			return ERR_PTR(-ENOMEM);
> +	}
> +
>   	if (!(inode_state_read_once(inode) & I_NEW)) {
>   		ret = check_igot_inode(inode, flags, function, line);
>   		if (ret) {
> diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
> index 982a1f831e22..21b5670d8503 100644
> --- a/fs/ext4/xattr.c
> +++ b/fs/ext4/xattr.c
> @@ -1550,7 +1550,7 @@ ext4_xattr_inode_cache_find(struct inode *inode, const void *value,
>   
>   	while (ce) {
>   		ea_inode = ext4_iget(inode->i_sb, ce->e_value,
> -				     EXT4_IGET_EA_INODE);
> +				     EXT4_IGET_EA_INODE | EXT4_IGET_NOWAIT);
>   		if (IS_ERR(ea_inode))
>   			goto next_entry;
>   		ext4_xattr_inode_set_class(ea_inode);


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