btrfs: check we grabbed inode reference when allocating an ordered extent

btrfs: check we grabbed inode reference when allocating an ordered extent

When allocating an ordered extent we call igrab() to get a reference on
the inode and attach it to the ordered extent. For an ordered extent we
always must have an inode reference since we during its life cycle we
need to access the inode for several things like for example:

* Inserting the ordered extent right after allocating it, when calling
  insert_ordered_extent() - we need to lock the inode's ordered_tree_lock;

* In the bio submission path we need to add checksums to the ordered
  extent and we end up at btrfs_add_ordered_sum(), where again we need
  to grab the inode from the ordered extent to lock the inode's
  ordered_tree_lock;

* When finishing an ordered extent, at btrfs_finish_ordered_extent(), we
  need again to access its inode in order to lock the inode's
  ordered_tree_lock;

* Etc etc etc.

Everywhere we deal with an ordered extent we always expect its inode to
be not NULL, the only exception being btrfs_put_ordered_extent() where
we check if it's NULL before calling btrfs_add_delayed_iput(), even though
we have already assumed it's not NULL when calling the tracepoint
trace_btrfs_ordered_extent_put() since the tracepoint dereferences the
inode to extract its number and root without ever checking it's NULL.

The igrab() call can return NULL if the inode is about to be freed or is
being freed (its state has I_FREEING or I_WILL_FREE set), and that's why
there's such check at btrfs_put_ordered_extent(). The igrab() and NULL
check were introduced in commit 5fd02043553b ("Btrfs: finish ordered
extents in their own thread") but even back then we always needed and
assumed igrab() returned a non-NULL pointer, since for example when
removing an ordered extent, at btrfs_remove_ordered_extent(), we assumed
the inode pointer was not NULL in order to access the inode's ordered
extent tree.

In fact whenever we allocate an ordered extent we are holding an inode
reference and the inode is not being freed or going to be freed (which
happens in the final iput), and since we depend on the inode for the
life cycle of the ordered extent, just make ordered extent allocation
to fail in case igrab() returns NULL and trigger a warning, to make it
clear it's not expected. This allows to remove the confusing NULL inode
check at btrfs_put_ordered_extent().

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>