On Fri, Aug 04, 2023 at 12:14:08PM +0200, Christoph Hellwig wrote:
> FYI, I can reproduce this trivially locally, but even after spending a
> significant time with the trace I'm still puzzled at what is going
> on. I've started trying to make sense of the lockdep report about
> returning to userspace with s_umount held, originall locked in
> get_tree_bdev and am still missing how it could happen.
So in the old scheme:
s = alloc_super()
-> down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
and assume you're not finding an old one immediately afterwards you'd
-> spin_lock(&sb_lock)
static int set_bdev_super(struct super_block *s, void *data)
{
s->s_bdev = data;
s->s_dev = s->s_bdev->bd_dev;
s->s_bdi = bdi_get(s->s_bdev->bd_disk->bdi);
if (bdev_stable_writes(s->s_bdev))
s->s_iflags |= SB_I_STABLE_WRITES;
return 0;
}
-> spin_unlock(&sb_lock)
in the new scheme you're doing:
s = alloc_super()
-> down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
and assume you're not finding an old one immediately afterwards you'd
up_write(&s->s_umount);
error = setup_bdev_super(s, fc->sb_flags, fc);
-> spin_lock(&sb_lock);
sb->s_bdev = bdev;
sb->s_bdi = bdi_get(bdev->bd_disk->bdi);
if (bdev_stable_writes(bdev))
sb->s_iflags |= SB_I_STABLE_WRITES;
-> spin_unlock(&sb_lock);
down_write(&s->s_umount);
Which looks like the lock ordering here is changed?
