linux-kernel - Re: [PATCH v2 07/12] docs: path-lookup: i_op->follow_link replaced with i_op->get

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Date:   Mon, 19 Apr 2021 11:28:14 +1000
From:   NeilBrown <neilb@...e.de>
To:     Fox Chen <foxhlchen@...il.com>
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: Re: [PATCH v2 07/12] docs: path-lookup: i_op->follow_link replaced
 with i_op->get_link

On Tue, Mar 16 2021, Fox Chen wrote:

> follow_link has been replaced by get_link() which can be
> called in RCU mode.
>
> see commit: commit 6b2553918d8b ("replace ->follow_link() with
> new method that could stay in RCU mode")
>
> Signed-off-by: Fox Chen <foxhlchen@...il.com>

Reviewed-By: NeilBrown <neilb@...e.de>

Thanks,
NeilBrown


> ---
>  Documentation/filesystems/path-lookup.rst | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
> index af5c20fecfef..e6b6c43ff0f6 100644
> --- a/Documentation/filesystems/path-lookup.rst
> +++ b/Documentation/filesystems/path-lookup.rst
> @@ -1060,13 +1060,11 @@ filesystem cannot successfully get a reference in RCU-walk mode, it
>  must return ``-ECHILD`` and ``unlazy_walk()`` will be called to return to
>  REF-walk mode in which the filesystem is allowed to sleep.
>  
> -The place for all this to happen is the ``i_op->follow_link()`` inode
> -method.  In the present mainline code this is never actually called in
> -RCU-walk mode as the rewrite is not quite complete.  It is likely that
> -in a future release this method will be passed an ``inode`` pointer when
> -called in RCU-walk mode so it both (1) knows to be careful, and (2) has the
> -validated pointer.  Much like the ``i_op->permission()`` method we
> -looked at previously, ``->follow_link()`` would need to be careful that
> +The place for all this to happen is the ``i_op->get_link()`` inode
> +method. This is called both in RCU-walk and REF-walk. In RCU-walk the
> +``dentry*`` argument is NULL, ``->get_link()`` can return -ECHILD to drop out of
> +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
> -- 
> 2.30.2

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