Documentation/filesystems/path-lookup.txt

36 even stores into cachelines of common dentries). This is known as "rcu-walk"
203 Inodes are also rcu freed, so the seqcount lookup dentry's inode may also be
212 Path walking code now has two distinct modes, ref-walk and rcu-walk. ref-walk
216 walking is operating on each dentry. rcu-walk uses seqcount based dentry
218 shared data in the dentry or inode. rcu-walk can not be applied to all cases,
219 eg. if the filesystem must sleep or perform non trivial operations, rcu-walk
226 path string, rcu-walk uses a d_seq protected snapshot. When looking up a
246     +---------------------+   rcu-walk begins here, we note d_seq, check the
273 Taking a refcount on a dentry from rcu-walk mode, by taking its d_lock,
275 "dropping rcu" or dropping from rcu-walk into ref-walk mode.
285 requires ref-walk, then instead of restarting the walk, we attempt to drop rcu
290 The detailed design for rcu-walk is like this:
291 * LOOKUP_RCU is set in nd->flags, which distinguishes rcu-walk from ref-walk.
296   access d_ops and i_ops during rcu-walk.
311 * When the destination dentry is reached, drop rcu there (ie. take d_lock,
315   a better errno) to signal an rcu-walk failure.
317 The cases where rcu-walk cannot continue are:
321 It may be possible eventually to make following links rcu-walk aware.
340 The following table gives rcu lookup statistics for a few simple workloads
342 drop rcu that fail due to d_seq failure and requiring the entire path lookup
343 again. Other cases are successful rcu-drops that are required before the final
345 routine requiring rcu drop, permission for permission check requiring drop,
348      rcu-lookups     restart  nodentry          link  revalidate  permission
355 What this shows is that failed rcu-walk lookups, ie. ones that are restarted
360 Dropping from rcu-walk to ref-walk mean that we have encountered a dentry where
363 condition that can't be resolved in rcu-walk mode.  Ideally, we drop rcu-walk
367 Note that a graceful drop from rcu-walk mode due to something such as the
369 rcu-walk scheme, because some elements of the path may have been walked in
370 rcu-walk mode. The further we get from common path elements (such as cwd or