linux-kernel - Re: [PATCH] fs: Eliminate a local variable to make the code more clear

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Date:   Wed, 09 Sep 2020 07:54:44 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Hao Lee <haolee.swjtu@...il.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: Eliminate a local variable to make the code more clear

Hao Lee <haolee.swjtu@...il.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.

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").  


Eric