Re: [PATCH 2/3] btrfs: switch extent_buffer spinning_writers from atomic to int

[Nikolay Borisov <nborisov@suse.com>]

On 30.05.19 г. 19:31 ч., David Sterba wrote:
> The spinning_writers is either 0 or 1 and always updated under the lock,
> so we don't need the atomic_t semantics.
> 
> Signed-off-by: David Sterba <dsterba@suse.com>
> ---
>  fs/btrfs/extent_io.c  |  2 +-
>  fs/btrfs/extent_io.h  |  2 +-
>  fs/btrfs/locking.c    | 10 +++++-----
>  fs/btrfs/print-tree.c |  2 +-
>  4 files changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index 57b6de9df7c4..71ee9e976307 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -4842,7 +4842,7 @@ __alloc_extent_buffer(struct btrfs_fs_info *fs_info, u64 start,
>  	BUG_ON(len > MAX_INLINE_EXTENT_BUFFER_SIZE);
>  
>  #ifdef CONFIG_BTRFS_DEBUG
> -	atomic_set(&eb->spinning_writers, 0);
> +	eb->spinning_writers = 0;
>  	atomic_set(&eb->spinning_readers, 0);
>  	atomic_set(&eb->read_locks, 0);
>  	atomic_set(&eb->write_locks, 0);
> diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
> index 201da61dfc21..5616b96c365d 100644
> --- a/fs/btrfs/extent_io.h
> +++ b/fs/btrfs/extent_io.h
> @@ -187,7 +187,7 @@ struct extent_buffer {
>  	wait_queue_head_t read_lock_wq;
>  	struct page *pages[INLINE_EXTENT_BUFFER_PAGES];
>  #ifdef CONFIG_BTRFS_DEBUG
> -	atomic_t spinning_writers;
> +	int spinning_writers;
>  	atomic_t spinning_readers;
>  	atomic_t read_locks;
>  	atomic_t write_locks;
> diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
> index 5feb01147e19..270667627977 100644
> --- a/fs/btrfs/locking.c
> +++ b/fs/btrfs/locking.c
> @@ -15,19 +15,19 @@
>  #ifdef CONFIG_BTRFS_DEBUG
>  static void btrfs_assert_spinning_writers_get(struct extent_buffer *eb)
>  {
> -	WARN_ON(atomic_read(&eb->spinning_writers));
> -	atomic_inc(&eb->spinning_writers);
> +	WARN_ON(eb->spinning_writers);
> +	eb->spinning_writers++;
>  }
>  
>  static void btrfs_assert_spinning_writers_put(struct extent_buffer *eb)
>  {
> -	WARN_ON(atomic_read(&eb->spinning_writers) != 1);
> -	atomic_dec(&eb->spinning_writers);
> +	WARN_ON(eb->spinning_writers != 1);
> +	eb->spinning_writers--;
>  }
>  
>  static void btrfs_assert_no_spinning_writers(struct extent_buffer *eb)
>  {
> -	WARN_ON(atomic_read(&eb->spinning_writers));
> +	WARN_ON(eb->spinning_writers);
>  }

IMO longterm  it will be good if those debug functions contained
lockdep_assert_held_exclusive/read macros for posterity.

>  
>  static void btrfs_assert_spinning_readers_get(struct extent_buffer *eb)
> diff --git a/fs/btrfs/print-tree.c b/fs/btrfs/print-tree.c
> index 7cb4f1fbe043..c5cc435ed39a 100644
> --- a/fs/btrfs/print-tree.c
> +++ b/fs/btrfs/print-tree.c
> @@ -157,7 +157,7 @@ static void print_eb_refs_lock(struct extent_buffer *eb)
>  		   atomic_read(&eb->read_locks),
>  		   eb->blocking_writers,
>  		   atomic_read(&eb->blocking_readers),
> -		   atomic_read(&eb->spinning_writers),
> +		   eb->spinning_writers,
>  		   atomic_read(&eb->spinning_readers),
>  		   eb->lock_owner, current->pid);
>  #endif
>