Re: [syzbot] [kernfs?] possible deadlock in kernfs_fop_llseek

[Al Viro <viro@zeniv.linux.org.uk>]

On Thu, Apr 04, 2024 at 12:33:40PM +0300, Amir Goldstein wrote:

> We do not (anymore) lock ovl inode in ovl_llseek(), see:
> b1f9d3858f72 ovl: use ovl_inode_lock in ovl_llseek()
> but ovl inode is held in operations (e.g. ovl_rename)
> which trigger copy up and call vfs_llseek() on the lower file.

OK, but why do we bother with ovl_inode_lock() there?
Note that serialization on struct file level is provided
on syscall level - see call of fdget_pos() in there.
IOW, which object are you protecting?  If it's struct file
passed your way, you should already have the serialization.
If it's underlying file on disk, that's up to vfs_llseek().
Exclusion with copyup by a different operation?

I'm not saying it's wrong - it's just that the thing is
tricky enough, so some clarification might be a good idea.