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SubjectRe: [PATCH 02/19] fs: don't take the i_lock in inode_inc_iversion
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On Wed, 2017-12-13 at 09:20 -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.
>
> Signed-off-by: Jeff Layton <jlayton@redhat.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@kernel.org>

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