fs: rcu-walk aware d_revalidate method

Require filesystems be aware of .d_revalidate being called in rcu-walk
mode (nd->flags & LOOKUP_RCU). For now do a simple push down, returning
-ECHILD from all implementations.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>

1 files changed, 9 insertions, 0 deletions

diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index 69b10ff5ec81..c936b4912383 100644
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -863,6 +863,15 @@ struct dentry_operations {
         dcache. Most filesystems leave this as NULL, because all their
         dentries in the dcache are valid
 
+        d_revalidate may be called in rcu-walk mode (nd->flags & LOOKUP_RCU).
+        If in rcu-walk mode, the filesystem must revalidate the dentry without
+        blocking or storing to the dentry, d_parent and d_inode should not be
+        used without care (because they can go NULL), instead nd->inode should
+        be used.
+
+        If a situation is encountered that rcu-walk cannot handle, return
+        -ECHILD and it will be called again in ref-walk mode.
+
   d_hash: called when the VFS adds a dentry to the hash table. The first
         dentry passed to d_hash is the parent directory that the name is
         to be hashed into. The inode is the dentry's inode.