linux-kernel - Re: [PATCH 04/16] vfs: do_last(): use inode variable

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Message-ID: <CAPa8GCAPCnPhuA7wErEry9OhL4CgegZkJTaptqVrFazF9kDBPA@mail.gmail.com>
Date:	Wed, 9 May 2012 09:57:29 +1000
From:	Nick Piggin <npiggin@...il.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, hch@...radead.org,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH 04/16] vfs: do_last(): use inode variable

On 8 May 2012 00:28, Miklos Szeredi <miklos@...redi.hu> wrote:
> Nick Piggin <npiggin@...il.com> writes:
>
>> On 25 April 2012 22:44, Miklos Szeredi <miklos@...redi.hu> wrote:
>>> From: Miklos Szeredi <mszeredi@...e.cz>
>>>
>>> Use helper variable instead of path->dentry->d_inode before complete_walk().
>>> This will allow this code to be used in RCU mode.
>>
>> What do you mean, allow it to be used?
>
> I mean allow the code to be shared between RCU and non-RCU mode.  See
> 10/16.
>
>>
>>>
>>> Signed-off-by: Miklos Szeredi <mszeredi@...e.cz>
>>> ---
>>>  fs/namei.c |    7 ++++---
>>>  1 files changed, 4 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/fs/namei.c b/fs/namei.c
>>> index 46d4bf6..f21ddb3 100644
>>> --- a/fs/namei.c
>>> +++ b/fs/namei.c
>>> @@ -2360,15 +2360,16 @@ static struct file *do_last(struct nameidata *nd, struct path *path,
>>>        if (error)
>>>                nd->flags |= LOOKUP_JUMPED;
>>>
>>> +       inode = path->dentry->d_inode ;
>>>        error = -ENOENT;
>>> -       if (!path->dentry->d_inode)
>>> +       if (!inode)
>>>                goto exit_dput;
>>>
>>> -       if (path->dentry->d_inode->i_op->follow_link)
>>> +       if (inode->i_op->follow_link)
>>>                return NULL;
>>>
>>>        path_to_nameidata(path, nd);
>>> -       nd->inode = path->dentry->d_inode;
>>> +       nd->inode = inode;
>>>        /* Why this, you ask?  _Now_ we might have grown LOOKUP_JUMPED... */
>>>        error = complete_walk(nd);
>>>        if (error)
>>
>> In rcu-walk mode, dentry->d_inode should not be accessed at all,
>> outside of the core lookup code that (should) have the correct
>> barriers and sequence locks.
>>
>> That logic should not escape into here, so I'm just not sure what
>> you're doing here.
>
> Right, dentry->d_inode is *not* going to be dereferenced in RCU mode.
> In RCU mode it will jump to the place just after the "inode =
> path->dentry->d_inode;" line.  See patch 10/16.

Hmm, OK, I admittedly didn't apply all patches and look at the result.

I wonder if that's getting too hairy... I'm sure consolidation is worthwhile
though. Perhaps comment or even a BUG_ON to ensure it is not in
rcu-walk mode, at points where you load the inode?
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