From 71e0a67dc6c26018e27fe0c670e2db023aa72d22 Mon Sep 17 00:00:00 2001 From: Fox Chen Date: Thu, 27 May 2021 17:16:09 +0800 Subject: [PATCH] docs: path-lookup: update do_last() part traling_symlink() was merged into lookup_last, do_last(). do_last() has later been split into open_last_lookups() and do_open(). see related commit: commit c5971b8c6354 ("take post-lookup part of do_last() out of loop") Signed-off-by: Fox Chen Reviewed-by: NeilBrown Link: https://lore.kernel.org/r/20210527091618.287093-5-foxhlchen@gmail.com Signed-off-by: Jonathan Corbet --- Documentation/filesystems/path-lookup.rst | 41 +++++++++++------------ 1 file changed, 19 insertions(+), 22 deletions(-) diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst index 652d3284f178..2b0b33168067 100644 --- a/Documentation/filesystems/path-lookup.rst +++ b/Documentation/filesystems/path-lookup.rst @@ -495,11 +495,11 @@ This is important when unmounting a filesystem that is inaccessible, such as one provided by a dead NFS server. Finally ``path_openat()`` is used for the ``open()`` system call; it -contains, in support functions starting with "``do_last()``", all the +contains, in support functions starting with "``open_last_lookups()``", all the complexity needed to handle the different subtleties of O_CREAT (with or without O_EXCL), final "``/``" characters, and trailing symbolic links. We will revisit this in the final part of this series, which -focuses on those symbolic links. "``do_last()``" will sometimes, but +focuses on those symbolic links. "``open_last_lookups()``" will sometimes, but not always, take ``i_rwsem``, depending on what it finds. Each of these, or the functions which call them, need to be alert to @@ -1196,29 +1196,26 @@ potentially need to call ``link_path_walk()`` again and again on successive symlinks until one is found that doesn't point to another symlink. -This case is handled by the relevant caller of ``link_path_walk()``, such as -``path_lookupat()`` using a loop that calls ``link_path_walk()``, and then -handles the final component. If the final component is a symlink -that needs to be followed, then ``trailing_symlink()`` is called to set -things up properly and the loop repeats, calling ``link_path_walk()`` -again. This could loop as many as 40 times if the last component of -each symlink is another symlink. +This case is handled by relevant callers of ``link_path_walk()``, such as +``path_lookupat()``, ``path_openat()`` using a loop that calls ``link_path_walk()``, +and then handles the final component by calling ``open_last_lookups()`` or +``lookup_last()``. If it is a symlink that needs to be followed, +``open_last_lookups()`` or ``lookup_last()`` will set things up properly and +return the path so that the loop repeats, calling +``link_path_walk()`` again. This could loop as many as 40 times if the last +component of each symlink is another symlink. -The various functions that examine the final component and possibly -report that it is a symlink are ``lookup_last()``, ``mountpoint_last()`` -and ``do_last()``, each of which use the same convention as -``walk_component()`` of returning ``1`` if a symlink was found that needs -to be followed. +Of the various functions that examine the final component, +``open_last_lookups()`` is the most interesting as it works in tandem +with ``do_open()`` for opening a file. Part of ``open_last_lookups()`` runs +with ``i_rwsem`` held and this part is in a separate function: ``lookup_open()``. -Of these, ``do_last()`` is the most interesting as it is used for -opening a file. Part of ``do_last()`` runs with ``i_rwsem`` held and this -part is in a separate function: ``lookup_open()``. +Explaining ``open_last_lookups()`` and ``do_open()`` completely is beyond the scope +of this article, but a few highlights should help those interested in exploring +the code. -Explaining ``do_last()`` completely is beyond the scope of this article, -but a few highlights should help those interested in exploring the -code. - -1. Rather than just finding the target file, ``do_last()`` needs to open +1. Rather than just finding the target file, ``do_open()`` is used after + ``open_last_lookup()`` to open it. If the file was found in the dcache, then ``vfs_open()`` is used for this. If not, then ``lookup_open()`` will either call ``atomic_open()`` (if the filesystem provides it) to combine the final lookup with the open, or