Re: [PATCH] Btrfs: avoid unnecessarily locking inode when clearing a range

[Chris Mason <clm@fb.com>]

On 07/27/2017 02:52 PM, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> If the range being cleared was not marked for defrag and we are not
> about to clear the range from the defrag status, we don't need to
> lock and unlock the inode.
> 
> Signed-off-by: Filipe Manana <fdmanana@suse.com>

Thanks Filipe, looks like it goes all the way back to:

commit 47059d930f0e002ff851beea87d738146804726d
Author: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Date:   Thu Jul 3 18:22:07 2014 +0800

     Btrfs: make defragment work with nodatacow option

I can't see how the inode lock is required here.

Reviewed-by: Chris Mason <clm@fb.com>

-chris

> ---
>   fs/btrfs/inode.c | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index eb495e956d53..51c45c0a8553 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -1797,10 +1797,11 @@ static void btrfs_clear_bit_hook(void *private_data,
>   	u64 len = state->end + 1 - state->start;
>   	u32 num_extents = count_max_extents(len);
>   
> -	spin_lock(&inode->lock);
> -	if ((state->state & EXTENT_DEFRAG) && (*bits & EXTENT_DEFRAG))
> +	if ((state->state & EXTENT_DEFRAG) && (*bits & EXTENT_DEFRAG)) {
> +		spin_lock(&inode->lock);
>   		inode->defrag_bytes -= len;
> -	spin_unlock(&inode->lock);
> +		spin_unlock(&inode->lock);
> +	}
>   
>   	/*
>   	 * set_bit and clear bit hooks normally require _irqsave/restore
>