fs: don't take the i_lock in inode_inc_iversion

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

diff --git a/include/linux/iversion.h b/include/linux/iversion.h
index d61e063f7c75dabfdee0f7cd98bbd4b02a3fdcf2..f268828f9f7e2c617a5df6c8834b9727b3e67164 100644 (file)
--- a/include/linux/iversion.h
+++ b/include/linux/iversion.h
@@ -110,12 +110,13 @@ 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