kernfs: use i_lock to protect concurrent inode updates

The inode operations .permission() and .getattr() use the kernfs node
write lock but all that's needed is to keep the rb tree stable while
updating the inode attributes as well as protecting the update itself
against concurrent changes.

And .permission() is called frequently during path walks and can cause
quite a bit of contention between kernfs node operations and path
walks when the number of concurrent walks is high.

To change kernfs_iop_getattr() and kernfs_iop_permission() to take
the rw sem read lock instead of the write lock an additional lock is
needed to protect against multiple processes concurrently updating
the inode attributes and link count in kernfs_refresh_inode().

The inode i_lock seems like the sensible thing to use to protect these
inode attribute updates so use it in kernfs_refresh_inode().

Signed-off-by: Ian Kent <raven@themaw.net>
2 files changed