parallel lookups: actual switch to rwsem

[linux-2.6-block.git] / Documentation / filesystems / porting

diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting
index 8810e2367fe6252cad7c8a4536d0d69a6eb2075b..1567a53857bdee03898fed5d47eec7a41f0ed0b3 100644 (file)
--- a/Documentation/filesystems/porting
+++ b/Documentation/filesystems/porting
@@ -539,3 +539,21 @@ in your dentry operations instead.
        it's a symlink.  Checking ->i_mode is really needed now.  In-tree we had
        to fix shmem_destroy_callback() that used to take that kind of shortcut;
        watch out, since that shortcut is no longer valid.
+--
+[mandatory]
+       ->i_mutex is replaced with ->i_rwsem now.  inode_lock() et.al. work as
+       they used to - they just take it exclusive.  However, ->lookup() may be
+       called with parent locked shared.  Its instances must not
+               * use d_instantiate) and d_rehash() separately - use d_add() or
+                 d_splice_alias() instead.
+               * use d_rehash() alone - call d_add(new_dentry, NULL) instead.
+               * in the unlikely case when (read-only) access to filesystem
+                 data structures needs exclusion for some reason, arrange it
+                 yourself.  None of the in-tree filesystems needed that.
+               * rely on ->d_parent and ->d_name not changing after dentry has
+                 been fed to d_add() or d_splice_alias().  Again, none of the
+                 in-tree instances relied upon that.
+       We are guaranteed that lookups of the same name in the same directory
+       will not happen in parallel ("same" in the sense of your ->d_compare()).
+       Lookups on different names in the same directory can and do happen in
+       parallel now.