Re: [PATCH v2] erofs: fix lockdep false positives on initializing erofs_pseudo_mnt

From: Baokun Li
Date: Thu Mar 07 2024 - 20:04:58 EST


On 2024/3/7 22:18, Gao Xiang wrote:


On 2024/3/7 18:10, Baokun Li wrote:
Lockdep reported the following issue when mounting erofs with a domain_id:

============================================
WARNING: possible recursive locking detected
6.8.0-rc7-xfstests #521 Not tainted
--------------------------------------------
mount/396 is trying to acquire lock:
ffff907a8aaaa0e0 (&type->s_umount_key#50/1){+.+.}-{3:3},
                        at: alloc_super+0xe3/0x3d0

but task is already holding lock:
ffff907a8aaa90e0 (&type->s_umount_key#50/1){+.+.}-{3:3},
                        at: alloc_super+0xe3/0x3d0

other info that might help us debug this:
  Possible unsafe locking scenario:

        CPU0
        ----
   lock(&type->s_umount_key#50/1);
   lock(&type->s_umount_key#50/1);

  *** DEADLOCK ***

  May be due to missing lock nesting notation

2 locks held by mount/396:
  #0: ffff907a8aaa90e0 (&type->s_umount_key#50/1){+.+.}-{3:3},
            at: alloc_super+0xe3/0x3d0
  #1: ffffffffc00e6f28 (erofs_domain_list_lock){+.+.}-{3:3},
            at: erofs_fscache_register_fs+0x3d/0x270 [erofs]

stack backtrace:
CPU: 1 PID: 396 Comm: mount Not tainted 6.8.0-rc7-xfstests #521
Call Trace:
  <TASK>
  dump_stack_lvl+0x64/0xb0
  validate_chain+0x5c4/0xa00
  __lock_acquire+0x6a9/0xd50
  lock_acquire+0xcd/0x2b0
  down_write_nested+0x45/0xd0
  alloc_super+0xe3/0x3d0
  sget_fc+0x62/0x2f0
  vfs_get_super+0x21/0x90
  vfs_get_tree+0x2c/0xf0
  fc_mount+0x12/0x40
  vfs_kern_mount.part.0+0x75/0x90
  kern_mount+0x24/0x40
  erofs_fscache_register_fs+0x1ef/0x270 [erofs]
  erofs_fc_fill_super+0x213/0x380 [erofs]

This is because the file_system_type of both erofs and the pseudo-mount
point of domain_id is erofs_fs_type, so two successive calls to
alloc_super() are considered to be using the same lock and trigger the
warning above.

Therefore add a nodev file_system_type called erofs_anon_fs_type in
fscache.c to silence this complaint. Because kern_mount() takes a
pointer to struct file_system_type, not its (string) name. So we don't
need to call register_filesystem(). In addition, call init_pseudo() in
erofs_anon_init_fs_context() as suggested by Al Viro, so that we can
remove erofs_fc_fill_pseudo_super(), erofs_fc_anon_get_tree(), and
erofs_anon_context_ops.

Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>

I will add

Suggested-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

when applying..
Okay, thanks for adding it.

Also since it's a false positive and too close to the
final so let's keep this patch in -next for a while and
then aim for v6.9-rc1 instead.

Thanks,
Gao Xiang
Fine! Thanks!
--
With Best Regards,
Baokun Li
.