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

[Gao Xiang]

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..

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