linux-kernel - Re: [PATCH 1/3] nsfs: add evict callback into struct proc_ns

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Date:   Sun, 5 May 2019 00:08:17 +0800
From:   Wenbin Zeng <wenbin.zeng@...il.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     davem@...emloft.net, bfields@...ldses.org, jlayton@...nel.org,
        trond.myklebust@...merspace.com, anna.schumaker@...app.com,
        wenbinzeng@...cent.com, dsahern@...il.com,
        nicolas.dichtel@...nd.com, willy@...radead.org,
        edumazet@...gle.com, jakub.kicinski@...ronome.com,
        tyhicks@...onical.com, chuck.lever@...cle.com, neilb@...e.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, linux-nfs@...r.kernel.org
Subject: Re: [PATCH 1/3] nsfs: add evict callback into struct
 proc_ns_operations

On Thu, May 02, 2019 at 04:04:06AM +0100, Al Viro wrote:
> On Wed, May 01, 2019 at 02:42:23PM +0800, Wenbin Zeng wrote:
> > The newly added evict callback shall be called by nsfs_evict(). Currently
> > only put() callback is called in nsfs_evict(), it is not able to release
> > all netns refcount, for example, a rpc client holds two netns refcounts,
> > these refcounts are supposed to be released when the rpc client is freed,
> > but the code to free rpc client is normally triggered by put() callback
> > only when netns refcount gets to 0, specifically:
> >     refcount=0 -> cleanup_net() -> ops_exit_list -> free rpc client
> > But netns refcount will never get to 0 before rpc client gets freed, to
> > break the deadlock, the code to free rpc client can be put into the newly
> > added evict callback.
> > 
> > Signed-off-by: Wenbin Zeng <wenbinzeng@...cent.com>
> > ---
> >  fs/nsfs.c               | 2 ++
> >  include/linux/proc_ns.h | 1 +
> >  2 files changed, 3 insertions(+)
> > 
> > diff --git a/fs/nsfs.c b/fs/nsfs.c
> > index 60702d6..5939b12 100644
> > --- a/fs/nsfs.c
> > +++ b/fs/nsfs.c
> > @@ -49,6 +49,8 @@ static void nsfs_evict(struct inode *inode)
> >  	struct ns_common *ns = inode->i_private;
> >  	clear_inode(inode);
> >  	ns->ops->put(ns);
> > +	if (ns->ops->evict)
> > +		ns->ops->evict(ns);
> 
> What's to guarantee that ns will not be freed by ->put()?
> Confused...

Hi Al, thank you very much. You are absolutely right.
->evict() should be called before ->put(), i.e.:

@@ -49,6 +49,8 @@ static void nsfs_evict(struct inode *inode)
	struct ns_common *ns = inode->i_private;
	clear_inode(inode);
+	if (ns->ops->evict)
+		ns->ops->evict(ns);
	ns->ops->put(ns);
 }

Does this look good?