Re: [PATCH v5 02/19] fs: don't take the i_lock in inode_inc_iversion

J. Bruce Fields
2018-01-18T21:45:35Z

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