linux-kernel - Re: [PATCH 02/19] fs: don't take the i_lock in inode_inc

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Message-ID: <1513201930.3498.37.camel@kernel.org>
Date:   Wed, 13 Dec 2017 16:52:10 -0500
From:   Jeff Layton <jlayton@...nel.org>
To:     linux-fsdevel@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, hch@....de, neilb@...e.de,
        bfields@...ldses.org, amir73il@...il.com, jack@...e.de,
        viro@...iv.linux.org.uk
Subject: Re: [PATCH 02/19] fs: don't take the i_lock in inode_inc_iversion

On Wed, 2017-12-13 at 09:20 -0500, Jeff Layton wrote:
> From: Jeff Layton <jlayton@...hat.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@...hat.com>
> ---
>  include/linux/fs.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 5001e77342fd..c234fac4bb77 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2136,9 +2136,9 @@ inode_set_iversion_queried(struct inode *inode, const 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;
>  }
>  

FWIW, I'm not sure this patch is strictly necessary as an interim step.

Adding the i_lock into the all of the places where we currently just do
inode->i_version++ without properly auditing all of them gave me pause
though.

In any case, the last patch in the series cleans this nastiness up.
-- 
Jeff Layton <jlayton@...nel.org>