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

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Message-ID: <87fwbcuku2.fsf@tucsk.pomaz.szeredi.hu>
Date:	Mon, 07 May 2012 16:28:21 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Nick Piggin <npiggin@...il.com>
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

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.

Thanks,
Miklos
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