On 22.11.18 г. 11:12 ч., Nikolay Borisov wrote:
>
>
> On 21.11.18 г. 20:59 ч., Josef Bacik wrote:
>> From: Josef Bacik <jba...@fb.com>
>>
>> We do this dance in cleanup_ref_head and check_ref_cleanup, unify it
>> into a helper and cleanup the calling functions.
>>
>> Signed-off-by: Josef Bacik <jba...@fb.com>
>> Reviewed-by: Omar Sandoval <osan...@fb.com>
>> ---
>> fs/btrfs/delayed-ref.c | 14 ++++++++++++++
>> fs/btrfs/delayed-ref.h | 3 ++-
>> fs/btrfs/extent-tree.c | 22 +++-------------------
>> 3 files changed, 19 insertions(+), 20 deletions(-)
>>
>> diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c
>> index 9301b3ad9217..b3e4c9fcb664 100644
>> --- a/fs/btrfs/delayed-ref.c
>> +++ b/fs/btrfs/delayed-ref.c
>> @@ -400,6 +400,20 @@ struct btrfs_delayed_ref_head *btrfs_select_ref_head(
>> return head;
>> }
>>
>> +void btrfs_delete_ref_head(struct btrfs_delayed_ref_root *delayed_refs,
>> + struct btrfs_delayed_ref_head *head)
>> +{
>> + lockdep_assert_held(&delayed_refs->lock);
>> + lockdep_assert_held(&head->lock);
>> +
>> + rb_erase_cached(&head->href_node, &delayed_refs->href_root);
>> + RB_CLEAR_NODE(&head->href_node);
>> + atomic_dec(&delayed_refs->num_entries);
>> + delayed_refs->num_heads--;
>> + if (head->processing == 0)
>> + delayed_refs->num_heads_ready--;
>
> num_heads_ready will never execute in cleanup_ref_head, since
> processing == 0 only when the ref head is unselected. Perhaps those 2
> lines shouldn't be in this function? I find it a bit confusing that if
> processing is 0 we decrement num_heads_ready in check_ref_cleanup, but
> in unselect_delayed_ref_head we set it to 0 and increment it.
>
>> +}
>> +
>> /*
>> * Helper to insert the ref_node to the tail or merge with tail.
>> *
>> diff --git a/fs/btrfs/delayed-ref.h b/fs/btrfs/delayed-ref.h
>> index 8e20c5cb5404..d2af974f68a1 100644
>> --- a/fs/btrfs/delayed-ref.h
>> +++ b/fs/btrfs/delayed-ref.h
>> @@ -261,7 +261,8 @@ static inline void btrfs_delayed_ref_unlock(struct
>> btrfs_delayed_ref_head *head)
>> {
>> mutex_unlock(&head->mutex);
>> }
>> -
>> +void btrfs_delete_ref_head(struct btrfs_delayed_ref_root *delayed_refs,
>> + struct btrfs_delayed_ref_head *head);
>>
>> struct btrfs_delayed_ref_head *btrfs_select_ref_head(
>> struct btrfs_delayed_ref_root *delayed_refs);
>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>> index d242a1174e50..c36b3a42f2bb 100644
>> --- a/fs/btrfs/extent-tree.c
>> +++ b/fs/btrfs/extent-tree.c
>> @@ -2474,12 +2474,9 @@ static int cleanup_ref_head(struct btrfs_trans_handle
>> *trans,
>> spin_unlock(&delayed_refs->lock);
>> return 1;
>> }
>> - delayed_refs->num_heads--;
>> - rb_erase_cached(&head->href_node, &delayed_refs->href_root);
>> - RB_CLEAR_NODE(&head->href_node);
>> + btrfs_delete_ref_head(delayed_refs, head);
>> spin_unlock(&head->lock);
>> spin_unlock(&delayed_refs->lock);
>> - atomic_dec(&delayed_refs->num_entries);
>>
>> trace_run_delayed_ref_head(fs_info, head, 0);
>>
>> @@ -6984,22 +6981,9 @@ static noinline int check_ref_cleanup(struct
>> btrfs_trans_handle *trans,
>> if (!mutex_trylock(&head->mutex))
>> goto out;
>>
>> - /*
>> - * at this point we have a head with no other entries. Go
>> - * ahead and process it.
>> - */
>> - rb_erase_cached(&head->href_node, &delayed_refs->href_root);
>> - RB_CLEAR_NODE(&head->href_node);
>> - atomic_dec(&delayed_refs->num_entries);
>> -
>> - /*
>> - * we don't take a ref on the node because we're removing it from the
>> - * tree, so we just steal the ref the tree was holding.
>> - */
>> - delayed_refs->num_heads--;
>> - if (head->processing == 0)
>> - delayed_refs->num_heads_ready--;
>> + btrfs_delete_ref_head(delayed_refs, head);
>> head->processing = 0;
On a closer inspection I think here we can do:
ASSERT(head->processing == 0) because at that point we've taken the
head->lock spinlock which is held during ordinary delayed refs
processing (in __btrfs_run_delayed_refs) when the head is selected (and
processing is 1). So head->processing == 0 here I think is a hard
invariant of the code. The decrement here should pair with the increment
when the head was initially added to the tree.
In cleanup_ref_head we don't need to ever worry about num_heads_ready
since it has already been decremented by btrfs_select_ref_head.
As a matter fact this counter is not used anywhere so we might as well
just remove it.
>
> Something is fishy here, before the code checked for processing == 0 and
> then also set it to 0 ?
>
>> +
>> spin_unlock(&head->lock);
>> spin_unlock(&delayed_refs->lock);
>>
>>
>
