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

[Baokun Li]

Hi Xiang,

On 2024/3/7 10:52, Gao Xiang wrote:
> Hi Baokun,
>
> On 2024/3/7 10:44, 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 named erofs_anon_fs_type to
> > silence this complaint. In addition, to reduce code coupling, refactor
> > out the erofs_anon_init_fs_context() and erofs_kill_pseudo_sb() 
> > functions
> > and move the erofs_pseudo_mnt related code to fscache.c.
> >
> > Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
>
> IMHO, in the beginning, I'd like to avoid introducing another fs type
> for erofs to share (meta)data between filesystems since it will cause
> churn, could we use some alternative way to resolve this?
>
> Or Jingbo might have some other ideas?
>
> Thanks,
> Gao Xiang

The usual way to avoid this kind of false positive is to add a subclass to
the lock, but s_umount is allocated, initialised and locked in 
alloc_super(),
so we can't find a place to set the subclass.

Alternatively, kern_mount(&erofs_fs_type) could be moved to
erofs_module_init() or erofs_fc_parse_param() to avoid s_umount nesting,
but that would have looked a bit strange.

So the final choice was to add a new file_system_type to avoid this false
positive. Since you don't like the idea of adding a new file_system_type,
do you think it would be ok to move kern_mount(&erofs_fs_type) to
erofs_module_init() or erofs_fc_parse_param()?

Thanks!
--
With Best Regards,
Baokun Li
.