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Date:   Thu, 18 Jan 2018 16:45:34 -0500
From:   "J. Bruce Fields" <bfields@fieldses.org>
To:     Jeff Layton <jlayton@kernel.org>
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        viro@zeniv.linux.org.uk, linux-nfs@vger.kernel.org, neilb@suse.de,
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Subject: Re: [PATCH v5 02/19] fs: don't take the i_lock in inode_inc_iversion
Message-ID: <20180118214534.GB5299@fieldses.org>
References: <20180109141059.25929-1-jlayton@kernel.org>
 <20180109141059.25929-3-jlayton@kernel.org>
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On Tue, Jan 09, 2018 at 09:10:42AM -0500, 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.

So I guess it's theoretically possible that e.g. if you read while it's
incrementing from 2^32-1 to 2^32 you could read 0, 1, or 2^32+1?

If so then you could see an i_version value reused and incorrectly
decide that a file hadn't changed.

But it's such a tiny case, and I think you convert this to atomic64_t
later anyway, so, whatever.

--b.

> 
> Signed-off-by: Jeff Layton <jlayton@redhat.com>
> ---
>  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