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Date:   Wed, 9 Sep 2020 15:32:43 +0000
From:   Hao Lee <haolee.swjtu@gmail.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Al Viro <viro@zeniv.linux.org.uk>, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] fs: Eliminate a local variable to make the code more
 clear
Message-ID: <20200909153243.GA25215@haolee.github.io>
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On Wed, Sep 09, 2020 at 07:54:44AM -0500, Eric W. Biederman wrote:
> Hao Lee <haolee.swjtu@gmail.com> writes:
> 
> > On Tue, Sep 08, 2020 at 07:48:57PM +0100, Al Viro wrote:
> >> On Tue, Sep 08, 2020 at 01:06:56PM +0000, Hao Lee wrote:
> >> > ping
> >> > 
> >> > On Wed, Jul 29, 2020 at 03:21:28PM +0000, Hao Lee wrote:
> >> > > The dentry local variable is introduced in 'commit 84d17192d2afd ("get
> >> > > rid of full-hash scan on detaching vfsmounts")' to reduce the length of
> >> > > some long statements for example
> >> > > mutex_lock(&path->dentry->d_inode->i_mutex). We have already used
> >> > > inode_lock(dentry->d_inode) to do the same thing now, and its length is
> >> > > acceptable. Furthermore, it seems not concise that assign path->dentry
> >> > > to local variable dentry in the statement before goto. So, this function
> >> > > would be more clear if we eliminate the local variable dentry.
> >> 
> >> How does it make the function more clear?  More specifically, what
> >> analysis of behaviour is simplified by that?
> >
> > When I first read this function, it takes me a few seconds to think
> > about if the local variable dentry is always equal to path->dentry and
> > want to know if it has special purpose. This local variable may confuse
> > other people too, so I think it would be better to eliminate it.
> 
> I tend to have the opposite reaction.  I read your patch and wonder
> why path->dentry needs to be reread what is changing path that I can not see.
> my back.

If I understand correctly, accessing path->dentry->d_inode needs one
more instruction than accessing dentry->d_inode, so the original code is
more efficient.

> 
> Now for clarity it would probably help to do something like:
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index bae0e95b3713..430f3b4785e3 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2206,7 +2206,7 @@ static struct mountpoint *lock_mount(struct path *path)
>                 return mp;
>         }
>         namespace_unlock();
> -       inode_unlock(path->dentry->d_inode);
> +       inode_unlock(dentry->d_inode);
>         path_put(path);
>         path->mnt = mnt;
>         dentry = path->dentry = dget(mnt->mnt_root);
> 
> 
> So at least the inode_lock and inode_unlock are properly paired.
> 
> At first glance inode_unlock using path->dentry instead of dentry
> appears to be an oversight in 84d17192d2af ("get rid of full-hash scan
> on detaching vfsmounts").  

I think I have understood why we use the local variable dentry. Thanks.

Regards,
Hao Lee

> 
> 
> Eric