FAILED: patch "[PATCH] ext4: fix ABBA deadlock in ext4_rmdir()" failed to apply to 6.6-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 19e16597f9baf1b5719a3a6c710486446f6808fd
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107053253-failed-6.6-19e16597f9ba@kernel.org' --subject-prefix 'PATCH 6.6.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 19e16597f9baf1b5719a3a6c710486446f6808fd Mon Sep 17 00:00:00 2001
From: Sergey Senozhatsky <senozhatsky@chromium.org>
Date: Thu, 27 Aug 2026 21:50:20 +0900
Subject: [PATCH] ext4: fix ABBA deadlock in ext4_rmdir()

We observe the following ABBA deadlock (casefolding enabled):

 INFO: task jbd2/dm-13-8:447 blocked for more than 720 seconds.
 task:jbd2/dm-13-8    state:D stack:0     pid:447   tgid:447   ppid:2
 Call Trace:
  <TASK>
  __schedule+0xc2c/0xe60
  schedule+0x40/0xe0
  jbd2_journal_wait_updates+0x8f/0xf0
  jbd2_journal_commit_transaction+0x321/0x1850
  kjournald2+0xa9/0x230
  kthread+0x226/0x2a0
  ret_from_fork+0x101/0x1e0
  ret_from_fork_asm+0x1a/0x30

 INFO: task Thread-10:4161 blocked for more than 720 seconds.
 task:Thread-10       state:D stack:0     pid:4161  tgid:3848  ppid:540
 Call Trace:
  <TASK>
  __schedule+0xc2c/0xe60
  schedule+0x40/0xe0
  wait_transaction_locked+0x8c/0xd0
  start_this_handle+0x252/0x870
  jbd2__journal_start+0x120/0x280
  __ext4_journal_start_sb+0x11a/0x250
  ext4_evict_inode+0x208/0x760
  iput+0x222/0x5c0
  dput+0x293/0x690
  ____fput+0x145/0x2b0
  task_work_run+0x7a/0xb0
  exit_to_user_mode_loop+0xc0/0xd0
  do_syscall_64+0x14b/0xf10
  entry_SYSCALL_64_after_hwframe+0x76/0x7e

 NMI backtrace for cpu 0
 CPU: 0 UID: 1010216 PID: 4160 Comm: Thread-7
 RIP: 0010:d_walk+0x0/0x290
[..]
 Call Trace:
 <TASK>
  shrink_dcache_parent+0xb2/0x100
  d_invalidate+0x50/0x110
  ext4_rmdir+0x3a2/0x3d0
  vfs_rmdir+0x9d/0x1d0
  do_rmdir+0xf0/0x330
  __x64_sys_unlinkat+0x34/0x50
  do_syscall_64+0x61/0xf10

The problem is that ext4_rmdir() calls d_invalidate() before stopping
the jbd2 transaction handle.  Because d_invalidate() traverses child
dentries it can encounter dying or in-use dentries.  If a concurrent
thread executing __dentry_kill() on a child dentry invokes ext4_evict_inode(),
it requests a new transaction handle via ext4_journal_start(), which
waits for all running handles to close.  However, the thread executing
ext4_rmdir() still holds its active transaction handle while looping
in shrink_dcache_parent() waiting for the dying child dentry to complete
eviction.

Stop transaction handle in ext4_rmdir() before calling d_invalidate().

Cc: stable@kernel.org
Fixes: b886ee3e778e ("ext4: Support case-insensitive file name lookups")
Co-developed-by: Sarthak Kukreti <sarthakkukreti@google.com>
Signed-off-by: Sarthak Kukreti <sarthakkukreti@google.com>
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Baokun Li <libaokun@linux.alibaba.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20260827125158.4030598-1-senozhatsky@chromium.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
---
 fs/ext4/namei.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index a6386c1d237f1..231a274b814ec 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -3209,19 +3209,20 @@ static int ext4_rmdir(struct inode *dir, struct dentry *dentry)
 	ext4_fc_track_unlink(handle, dentry);
 	retval = ext4_mark_inode_dirty(handle, dir);
 
+end_rmdir:
+	brelse(bh);
+	if (handle)
+		ext4_journal_stop(handle);
+
 	/* VFS negative dentries are incompatible with Encoding and
 	 * Case-insensitiveness. Eventually we'll want avoid
 	 * invalidating the dentries here, alongside with returning the
 	 * negative dentries at ext4_lookup(), when it is better
 	 * supported by the VFS for the CI case.
 	 */
-	if (IS_ENABLED(CONFIG_UNICODE) && IS_CASEFOLDED(dir))
+	if (!retval && IS_ENABLED(CONFIG_UNICODE) && IS_CASEFOLDED(dir))
 		d_invalidate(dentry);
 
-end_rmdir:
-	brelse(bh);
-	if (handle)
-		ext4_journal_stop(handle);
 	return retval;
 }
 
-- 
2.53.0