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docs: path-lookup: update do_last() part

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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 c5971b8c63 ("take post-lookup
part of do_last() out of loop")

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
Reviewed-by: NeilBrown <neilb@suse.de>
Link: https://lore.kernel.org/r/20210527091618.287093-5-foxhlchen@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Fox Chen 2021-05-27 17:16:09 +08:00 committed by Jonathan Corbet
parent 8593d2cc8c
commit 71e0a67dc6
1 changed files with 19 additions and 22 deletions
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41
Documentation/filesystems/path-lookup.rst
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@ -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