Re: Need advice with iput() deadlock during writeback

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

On Wed, Sep 17, 2025 at 10:39:22PM +0200, Mateusz Guzik wrote:

> Linux has to have something of the sort for dentries, otherwise the
> current fput stuff would not be safe. I find it surprising to learn
> inodes are treated differently.

If you are looking at vnode counterparts, dentries are closer to that.
Inodes are secondary.

And no, it's not a "wait for references to go away" - every file holds
a _pair_ of references, one to mount and another to dentry.

Additional references to mount => umount() gets -EBUSY, lazy umount()
(with MNT_DETACH) gets the sucker removed from the mount tree, with
shutdown deferred (at least) until the last reference to mount goes away.

Once the mount refcount hits zero and the damn thing gets taken apart,
an active reference to superblock (i.e. to filesystem instance) is
dropped.

If that was not the last one (e.g. it's mounted elsewhere as well), we
are not waiting for anything.  If it *was* the last active ref, we
shut the filesystem instance down; that's _it_ - once you are into
->kill_sb(), it's all over.

Linux VFS is seriously different from Heidemann's-derived ones you'll find in
BSD land these days.  Different taxonomy of objects, among other things...