Re: [PATCH v2 01/34] bdev: open block device as files

Jan Kara
2024-03-14T16:58:20Z

On Thu 14-03-24 15:47:52, Christian Brauner wrote:
> On Thu, Mar 14, 2024 at 12:10:59PM +0100, Christian Brauner wrote:
> > On Tue, Mar 12, 2024 at 07:32:35PM -0700, Christoph Hellwig wrote:
> > > Now that this is in mainline it seems to cause blktests to crash
> > > nbd/003 with a rather non-obvious oops for me:
> >
> > Ok, will be looking into that next.
>
> Ok, I know what's going on. Basically, fput() on the block device runs
> asynchronously which means that bdev->bd_holder can still be set to @sb
> after it has already been freed. Let me illustrate what I mean:
>
> P1 P2
> mount(sb) fd = open("/dev/nbd", ...)
> -> file = bdev_file_open_by_dev(..., sb, ...)
> bdev->bd_holder = sb;
>
> // Later on:
>
> umount(sb)
> ->kill_block_super(sb)
> |----> [fput() -> deferred via task work]
> -> put_super(sb) -> frees the sb via rcu
> |
> | nbd_ioctl(NBD_CLEAR_SOCK)
> | -> disk_force_media_change()
> | -> bdev_mark_dead()
> | -> fs_mark_dead()
> | // Finds bdev->bd_holder == sb
> |-> file->release::bdev_release() // Tries to get reference to it but it's freed; frees it again
> bdev->bd_holder = NULL;
>
> Two solutions that come to my mind:
>
> [1] Indicate to fput() that this is an internal block devices open and
> thus just close it synchronously. This is fine. First, because the block
> device superblock is never unmounted or anything so there's no risk
> from hanging there for any reason. Second, bdev_release() also ran
> synchronously so any issue that we'd see from calling
> file->f_op->release::bdev_release() we would have seen from
> bdev_release() itself. See [1.1] for a patch.
>
> (2) Take a temporary reference to the holder when opening the block
> device. This is also fine afaict because we differentiate between
> passive and active references on superblocks and what we already do
> in fs_bdev_mark_dead() and friends. This mean we don't have to mess
> around with fput(). See [1.2] for a patch.
>
> (3) Revert and cry. No patch.
>
> Personally, I think (2) is more desirable because we don't lose the
> async property of fput() and we don't need to have a new FMODE_* flag.
> I'd like to do some more testing with this. Thoughts?

Yeah, 2) looks like the least painful solution to me as well.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR