Message-Id: <20210527091618.287093-9-foxhlchen@gmail.com>
Date:   Thu, 27 May 2021 17:16:13 +0800
From:   Fox Chen <foxhlchen@...il.com>
To:     neilb@...e.de
Cc:     Fox Chen <foxhlchen@...il.com>, corbet@....net,
        vegard.nossum@...cle.com, viro@...iv.linux.org.uk,
        rdunlap@...radead.org, grandmaster@...klimov.de,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        gregkh@...uxfoundation.org
Subject: [PATCH v3 08/13] docs: path-lookup: update i_op->put_link and cookie description

No inode->put_link operation anymore. We use delayed_call to
deal with link destruction. Cookie has been replaced with
struct delayed_call.

Related commit: commit fceef393a538 ("switch ->get_link() to
delayed_call, kill ->put_link()")

Signed-off-by: Fox Chen <foxhlchen@...il.com>
---
 Documentation/filesystems/path-lookup.rst | 30 ++++++-----------------
 1 file changed, 8 insertions(+), 22 deletions(-)

diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
index 4650c6427963..3855809784cf 100644
--- a/Documentation/filesystems/path-lookup.rst
+++ b/Documentation/filesystems/path-lookup.rst
@@ -1066,34 +1066,20 @@ method. This is called both in RCU-walk and REF-walk. In RCU-walk the
 RCU-walk.  Much like the ``i_op->permission()`` method we
 looked at previously, ``->get_link()`` would need to be careful that
 all the data structures it references are safe to be accessed while
-holding no counted reference, only the RCU lock.  Though getting a
-reference with ``->follow_link()`` is not yet done in RCU-walk mode, the
-code is ready to release the reference when that does happen.
-
-This need to drop the reference to a symlink adds significant
-complexity.  It requires a reference to the inode so that the
-``i_op->put_link()`` inode operation can be called.  In REF-walk, that
-reference is kept implicitly through a reference to the dentry, so
-keeping the ``struct path`` of the symlink is easiest.  For RCU-walk,
-the pointer to the inode is kept separately.  To allow switching from
-RCU-walk back to REF-walk in the middle of processing nested symlinks
-we also need the seq number for the dentry so we can confirm that
-switching back was safe.
-
-Finally, when providing a reference to a symlink, the filesystem also
-provides an opaque "cookie" that must be passed to ``->put_link()`` so that it
-knows what to free.  This might be the allocated memory area, or a
-pointer to the ``struct page`` in the page cache, or something else
-completely.  Only the filesystem knows what it is.
+holding no counted reference, only the RCU lock. A callback
+``struct delayed_called`` will be passed to ``->get_link()``:
+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
+``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,
 whether in RCU-walk or REF-walk, the symlink stack needs to contain,
 along with the path remnants:
 
-- the ``struct path`` to provide a reference to the inode in REF-walk
-- the ``struct inode *`` to provide a reference to the inode in RCU-walk
+- the ``struct path`` to provide a reference to the previous path
+- the ``const char *`` to provide a reference to the to previous name
 - the ``seq`` to allow the path to be safely switched from RCU-walk to REF-walk
-- the ``cookie`` that tells ``->put_path()`` what to put.
+- the ``struct delayed_call`` for later invocation.
 
 This means that each entry in the symlink stack needs to hold five
 pointers and an integer instead of just one pointer (the path
-- 
2.31.1

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