From: Jan Kara <jack@suse.cz>
Subject: Re: [PATCH v5 02/19] fs: don't take the i_lock in inode_inc_iversion
Date: Tue, 9 Jan 2018 16:14:28 +0100
Message-ID: <20180109151428.yuhawmbgv7tywjcq@quack2.suse.cz>
References: <20180109141059.25929-1-jlayton@kernel.org>
 <20180109141059.25929-3-jlayton@kernel.org>
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To: Jeff Layton <jlayton@kernel.org>
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On Tue 09-01-18 09:10:42, Jeff Layton wrote:
> From: Jeff Layton <jlayton@redhat.com>
> 
> The rationale for taking the i_lock when incrementing this value is
> lost in antiquity. The readers of the field don't take it (at least
> not universally), so my assumption is that it was only done here to
> serialize incrementors.
> 
> If that is indeed the case, then we can drop the i_lock from this
> codepath and treat it as a atomic64_t for the purposes of
> incrementing it. This allows us to use inode_inc_iversion without
> any danger of lock inversion.
> 
> Note that the read side is not fetched atomically with this change.
> The assumption here is that that is not a critical issue since the
> i_version is not fully synchronized with anything else anyway.
> 
> Signed-off-by: Jeff Layton <jlayton@redhat.com>

This changes the memory barrier behavior but IMO it is good enough for an
intermediate version. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  include/linux/iversion.h | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/iversion.h b/include/linux/iversion.h
> index d09cc3a08740..5ad9eaa3a9b0 100644
> --- a/include/linux/iversion.h
> +++ b/include/linux/iversion.h
> @@ -104,12 +104,13 @@ inode_set_iversion_queried(struct inode *inode, u64 new)
>  static inline bool
>  inode_maybe_inc_iversion(struct inode *inode, bool force)
>  {
> -	spin_lock(&inode->i_lock);
> -	inode->i_version++;
> -	spin_unlock(&inode->i_lock);
> +	atomic64_t *ivp = (atomic64_t *)&inode->i_version;
> +
> +	atomic64_inc(ivp);
>  	return true;
>  }
>  
> +
>  /**
>   * inode_inc_iversion - forcibly increment i_version
>   * @inode: inode that needs to be updated
> -- 
> 2.14.3
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR