Commit 8943474a authored by Fox Chen's avatar Fox Chen Committed by Jonathan Corbet

docs: path-lookup: use bare function() rather than literals

As suggested by Matthew Wilcox and Jonathan Corbet, drop ``...``
literals around function names of this patchset.
Signed-off-by: default avatarFox Chen <foxhlchen@gmail.com>
Reviewed-by: default avatarNeilBrown <neilb@suse.de>
Link: https://lore.kernel.org/r/20210527091618.287093-14-foxhlchen@gmail.comSigned-off-by: default avatarJonathan Corbet <corbet@lwn.net>
parent ef4aa53f
...@@ -450,13 +450,13 @@ If that doesn't get a good result, it calls "``lookup_slow()``" which ...@@ -450,13 +450,13 @@ If that doesn't get a good result, it calls "``lookup_slow()``" which
takes ``i_rwsem``, rechecks the cache, and then asks the filesystem takes ``i_rwsem``, rechecks the cache, and then asks the filesystem
to find a definitive answer. to find a definitive answer.
As the last step of ``walk_component()``, ``step_into()`` will be called either As the last step of walk_component(), step_into() will be called either
directly from walk_component() or from handle_dots(). It calls directly from walk_component() or from handle_dots(). It calls
``handle_mounts()``, to check and handle mount points, in which a new handle_mounts(), to check and handle mount points, in which a new
``struct path`` is created containing a counted reference to the new dentry and ``struct path`` is created containing a counted reference to the new dentry and
a reference to the new ``vfsmount`` which is only counted if it is a reference to the new ``vfsmount`` which is only counted if it is
different from the previous ``vfsmount``. Then if there is different from the previous ``vfsmount``. Then if there is
a symbolic link, ``step_into()`` calls ``pick_link()`` to deal with it, a symbolic link, step_into() calls pick_link() to deal with it,
otherwise it installs the new ``struct path`` in the ``struct nameidata``, and otherwise it installs the new ``struct path`` in the ``struct nameidata``, and
drops the unneeded references. drops the unneeded references.
...@@ -472,8 +472,8 @@ Handling the final component ...@@ -472,8 +472,8 @@ Handling the final component
``nd->last_type`` to refer to the final component of the path. It does ``nd->last_type`` to refer to the final component of the path. It does
not call ``walk_component()`` that last time. Handling that final not call ``walk_component()`` that last time. Handling that final
component remains for the caller to sort out. Those callers are component remains for the caller to sort out. Those callers are
``path_lookupat()``, ``path_parentat()`` and path_lookupat(), path_parentat() and
``path_openat()`` each of which handles the differing requirements of path_openat() each of which handles the differing requirements of
different system calls. different system calls.
``path_parentat()`` is clearly the simplest - it just wraps a little bit ``path_parentat()`` is clearly the simplest - it just wraps a little bit
...@@ -489,17 +489,17 @@ object is wanted such as by ``stat()`` or ``chmod()``. It essentially just ...@@ -489,17 +489,17 @@ object is wanted such as by ``stat()`` or ``chmod()``. It essentially just
calls ``walk_component()`` on the final component through a call to calls ``walk_component()`` on the final component through a call to
``lookup_last()``. ``path_lookupat()`` returns just the final dentry. ``lookup_last()``. ``path_lookupat()`` returns just the final dentry.
It is worth noting that when flag ``LOOKUP_MOUNTPOINT`` is set, It is worth noting that when flag ``LOOKUP_MOUNTPOINT`` is set,
``path_lookupat()`` will unset LOOKUP_JUMPED in nameidata so that in the path_lookupat() will unset LOOKUP_JUMPED in nameidata so that in the
subsequent path traversal ``d_weak_revalidate()`` won't be called. subsequent path traversal d_weak_revalidate() won't be called.
This is important when unmounting a filesystem that is inaccessible, such as This is important when unmounting a filesystem that is inaccessible, such as
one provided by a dead NFS server. one provided by a dead NFS server.
Finally ``path_openat()`` is used for the ``open()`` system call; it Finally ``path_openat()`` is used for the ``open()`` system call; it
contains, in support functions starting with "``open_last_lookups()``", all the contains, in support functions starting with "open_last_lookups()", all the
complexity needed to handle the different subtleties of O_CREAT (with complexity needed to handle the different subtleties of O_CREAT (with
or without O_EXCL), final "``/``" characters, and trailing symbolic or without O_EXCL), final "``/``" characters, and trailing symbolic
links. We will revisit this in the final part of this series, which links. We will revisit this in the final part of this series, which
focuses on those symbolic links. "``open_last_lookups()``" will sometimes, but focuses on those symbolic links. "open_last_lookups()" will sometimes, but
not always, take ``i_rwsem``, depending on what it finds. not always, take ``i_rwsem``, depending on what it finds.
Each of these, or the functions which call them, need to be alert to Each of these, or the functions which call them, need to be alert to
...@@ -651,9 +651,9 @@ RCU-walk finds it cannot stop gracefully, it simply gives up and ...@@ -651,9 +651,9 @@ RCU-walk finds it cannot stop gracefully, it simply gives up and
restarts from the top with REF-walk. restarts from the top with REF-walk.
This pattern of "try RCU-walk, if that fails try REF-walk" can be This pattern of "try RCU-walk, if that fails try REF-walk" can be
clearly seen in functions like ``filename_lookup()``, clearly seen in functions like filename_lookup(),
``filename_parentat()``, filename_parentat(),
``do_filp_open()``, and ``do_file_open_root()``. These four do_filp_open(), and do_file_open_root(). These four
correspond roughly to the three ``path_*()`` functions we met earlier, correspond roughly to the three ``path_*()`` functions we met earlier,
each of which calls ``link_path_walk()``. The ``path_*()`` functions are each of which calls ``link_path_walk()``. The ``path_*()`` functions are
called using different mode flags until a mode is found which works. called using different mode flags until a mode is found which works.
...@@ -1069,8 +1069,8 @@ all the data structures it references are safe to be accessed while ...@@ -1069,8 +1069,8 @@ all the data structures it references are safe to be accessed while
holding no counted reference, only the RCU lock. A callback holding no counted reference, only the RCU lock. A callback
``struct delayed_called`` will be passed to ``->get_link()``: ``struct delayed_called`` will be passed to ``->get_link()``:
file systems can set their own put_link function and argument through file systems can set their own put_link function and argument through
``set_delayed_call()``. Later on, when VFS wants to put link, it will call set_delayed_call(). Later on, when VFS wants to put link, it will call
``do_delayed_call()`` to invoke that callback function with the argument. do_delayed_call() to invoke that callback function with the argument.
In order for the reference to each symlink to be dropped when the walk completes, In order for the reference to each symlink to be dropped when the walk completes,
whether in RCU-walk or REF-walk, the symlink stack needs to contain, whether in RCU-walk or REF-walk, the symlink stack needs to contain,
...@@ -1103,7 +1103,7 @@ doesn't need to notice. Getting this ``name`` variable on and off the ...@@ -1103,7 +1103,7 @@ doesn't need to notice. Getting this ``name`` variable on and off the
stack is very straightforward; pushing and popping the references is stack is very straightforward; pushing and popping the references is
a little more complex. a little more complex.
When a symlink is found, ``walk_component()`` calls ``pick_link()`` via ``step_into()`` When a symlink is found, walk_component() calls pick_link() via step_into()
which returns the link from the filesystem. which returns the link from the filesystem.
Providing that operation is successful, the old path ``name`` is placed on the Providing that operation is successful, the old path ``name`` is placed on the
stack, and the new value is used as the ``name`` for a while. When the end of stack, and the new value is used as the ``name`` for a while. When the end of
...@@ -1136,10 +1136,10 @@ Symlinks with no final component ...@@ -1136,10 +1136,10 @@ Symlinks with no final component
A pair of special-case symlinks deserve a little further explanation. A pair of special-case symlinks deserve a little further explanation.
Both result in a new ``struct path`` (with mount and dentry) being set Both result in a new ``struct path`` (with mount and dentry) being set
up in the ``nameidata``, and result in ``pick_link()`` returning ``NULL``. up in the ``nameidata``, and result in pick_link() returning ``NULL``.
The more obvious case is a symlink to "``/``". All symlinks starting The more obvious case is a symlink to "``/``". All symlinks starting
with "``/``" are detected in ``pick_link()`` which resets the ``nameidata`` with "``/``" are detected in pick_link() which resets the ``nameidata``
to point to the effective filesystem root. If the symlink only to point to the effective filesystem root. If the symlink only
contains "``/``" then there is nothing more to do, no components at all, contains "``/``" then there is nothing more to do, no components at all,
so ``NULL`` is returned to indicate that the symlink can be released and so ``NULL`` is returned to indicate that the symlink can be released and
...@@ -1157,9 +1157,9 @@ target file, not just the name of it. When you ``readlink`` these ...@@ -1157,9 +1157,9 @@ target file, not just the name of it. When you ``readlink`` these
objects you get a name that might refer to the same file - unless it objects you get a name that might refer to the same file - unless it
has been unlinked or mounted over. When ``walk_component()`` follows has been unlinked or mounted over. When ``walk_component()`` follows
one of these, the ``->get_link()`` method in "procfs" doesn't return one of these, the ``->get_link()`` method in "procfs" doesn't return
a string name, but instead calls ``nd_jump_link()`` which updates the a string name, but instead calls nd_jump_link() which updates the
``nameidata`` in place to point to that target. ``->get_link()`` then ``nameidata`` in place to point to that target. ``->get_link()`` then
returns ``NULL``. Again there is no final component and ``pick_link()`` returns ``NULL``. Again there is no final component and pick_link()
returns ``NULL``. returns ``NULL``.
Following the symlink in the final component Following the symlink in the final component
...@@ -1177,35 +1177,35 @@ potentially need to call ``link_path_walk()`` again and again on ...@@ -1177,35 +1177,35 @@ potentially need to call ``link_path_walk()`` again and again on
successive symlinks until one is found that doesn't point to another successive symlinks until one is found that doesn't point to another
symlink. symlink.
This case is handled by relevant callers of ``link_path_walk()``, such as 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()``, path_lookupat(), path_openat() using a loop that calls link_path_walk(),
and then handles the final component by calling ``open_last_lookups()`` or and then handles the final component by calling open_last_lookups() or
``lookup_last()``. If it is a symlink that needs to be followed, lookup_last(). If it is a symlink that needs to be followed,
``open_last_lookups()`` or ``lookup_last()`` will set things up properly and open_last_lookups() or lookup_last() will set things up properly and
return the path so that the loop repeats, calling return the path so that the loop repeats, calling
``link_path_walk()`` again. This could loop as many as 40 times if the last link_path_walk() again. This could loop as many as 40 times if the last
component of each symlink is another symlink. component of each symlink is another symlink.
Of the various functions that examine the final component, Of the various functions that examine the final component,
``open_last_lookups()`` is the most interesting as it works in tandem 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 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()``. 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 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 of this article, but a few highlights should help those interested in exploring
the code. the code.
1. Rather than just finding the target file, ``do_open()`` is used after 1. Rather than just finding the target file, do_open() is used after
``open_last_lookup()`` to open open_last_lookup() to open
it. If the file was found in the dcache, then ``vfs_open()`` is used for 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 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 the filesystem provides it) to combine the final lookup with the open, or
will perform the separate ``i_op->lookup()`` and ``i_op->create()`` steps will perform the separate ``i_op->lookup()`` and ``i_op->create()`` steps
directly. In the later case the actual "open" of this newly found or directly. In the later case the actual "open" of this newly found or
created file will be performed by ``vfs_open()``, just as if the name created file will be performed by vfs_open(), just as if the name
were found in the dcache. were found in the dcache.
2. ``vfs_open()`` can fail with ``-EOPENSTALE`` if the cached information 2. vfs_open() can fail with ``-EOPENSTALE`` if the cached information
wasn't quite current enough. If it's in RCU-walk ``-ECHILD`` will be returned wasn't quite current enough. If it's in RCU-walk ``-ECHILD`` will be returned
otherwise ``-ESTALE`` is returned. When ``-ESTALE`` is returned, the caller may otherwise ``-ESTALE`` is returned. When ``-ESTALE`` is returned, the caller may
retry with ``LOOKUP_REVAL`` flag set. retry with ``LOOKUP_REVAL`` flag set.
...@@ -1218,8 +1218,8 @@ the code. ...@@ -1218,8 +1218,8 @@ the code.
will create a file called ``/tmp/bar``. This is not permitted if will create a file called ``/tmp/bar``. This is not permitted if
``O_EXCL`` is set but otherwise is handled for an O_CREAT open much ``O_EXCL`` is set but otherwise is handled for an O_CREAT open much
like for a non-creating open: ``lookup_last()`` or ``open_last_lookup()`` like for a non-creating open: lookup_last() or open_last_lookup()
returns a non ``NULL`` value, and ``link_path_walk()`` gets called and the returns a non ``NULL`` value, and link_path_walk() gets called and the
open process continues on the symlink that was found. open process continues on the symlink that was found.
Updating the access time Updating the access time
......
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