linux-kernel - Re: [PATCH v3] SUNRPC: set desired file system root before connecting local transports

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Date:	Tue, 9 Oct 2012 21:23:36 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"Myklebust, Trond" <Trond.Myklebust@...app.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Stanislav Kinsbursky <skinsbursky@...allels.com>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...nvz.org" <devel@...nvz.org>
Subject: Re: [PATCH v3] SUNRPC: set desired file system root before
 connecting local transports

On Tue, Oct 09, 2012 at 03:47:42PM -0700, Eric W. Biederman wrote:
> "J. Bruce Fields" <bfields@...ldses.org> writes:
> 
> > On Tue, Oct 09, 2012 at 01:20:48PM -0700, Eric W. Biederman wrote:
> >> "Myklebust, Trond" <Trond.Myklebust@...app.com> writes:
> >> 
> >> > On Tue, 2012-10-09 at 15:35 -0400, J. Bruce Fields wrote:
> >> >> Cc'ing Eric since I seem to recall he suggested doing it this way?
> >> 
> >> Yes.  On second look setting fs->root won't work. We need to change fs.
> >> The problem is that by default all kernel threads share fs so changing
> >> fs->root will have non-local consequences.
> >
> > Oh, huh.  And we can't "unshare" it somehow?
> 
> I don't fully understand how nfs uses kernel threads and work queues.
> My general understanding is work queues reuse their kernel threads
> between different users.  So it is mostly a don't pollute your
> environment thing.  If there was a dedicated kernel thread for each
> environment this would be trivial.
> 
> What I was suggesting here is changing task->fs instead of
> task->fs.root.  That should just require task_lock().

Oh, OK, got it--if that works, great.

> > Sorry, I don't know much about devtmpfs, are you suggesting it as a
> > model?  What exactly should we look at?
> 
> Roughly all I meant was that devtmpsfsd is a kernel thread that runs
> with an unshared fs struct.  Although I admit devtmpfsd is for all
> practical purposes a userspace daemon that just happens to run in kernel
> space.

Thanks for the explanation.

--b.
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