Re: possible deadlock in generic_file_write_iter

[Al Viro]

On Sun, Nov 05, 2017 at 02:25:00AM -0800, syzbot wrote:

> loop0/2986 is trying to acquire lock:
>  (&sb->s_type->i_mutex_key#9){++++}, at: [<ffffffff8186f9ec>] inode_lock
> include/linux/fs.h:712 [inline]
>  (&sb->s_type->i_mutex_key#9){++++}, at: [<ffffffff8186f9ec>]
> generic_file_write_iter+0xdc/0x7a0 mm/filemap.c:3151
> 
> but now in release context of a crosslock acquired at the following:
>  ((complete)&ret.event){+.+.}, at: [<ffffffff822a055e>]
> submit_bio_wait+0x15e/0x200 block/bio.c:953
> 
> which lock already depends on the new lock.

Almost certainly a false positive...  lockdep can't tell ->i_rwsem of
inode on filesystem that lives on /dev/loop0 and that of inode of
the backing file of /dev/loop0.

Try and put them on different filesystem types and see if you still
can reproduce that.  We do have a partial ordering between the filesystems,
namely "(parts of) hosting device of X live in a file on Y".  It's
going to be acyclic, or you have a much worse problem.  And that's
what really orders the things here.