linux-kernel - Re: [PATCH 0/2] NFSD: fix races in service per-net resources allocation

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Date:	Mon, 11 Feb 2013 11:37:15 -0500
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Stanislav Kinsbursky <skinsbursky@...allels.com>
Cc:	akpm@...ux-foundation.org, linux-nfs@...r.kernel.org,
	Trond.Myklebust@...app.com, linux-kernel@...r.kernel.org,
	devel@...nvz.org
Subject: Re: [PATCH 0/2] NFSD: fix races in service per-net resources
 allocation

On Mon, Feb 11, 2013 at 10:18:18AM +0400, Stanislav Kinsbursky wrote:
> This one looks a bit complicated and confusing to me. Probably because
> I'm not that familiar with service transports processing logic.  So,
> as I can see, we now try to run over all per-net pool-assigned
> transports, remove them from "ready" queue and delete one by one.
> Then we try to enqueue all temporary sockets. But where in enqueueing
> of permanent sockets? I.e. how does they be destroyed with this patch?
> Then we once again try to run over all per-net pool-assigned
> transports, remove them from "ready" queue and delete one by one.  Why
> twice? I.e. why not just lose them, then enqueue them and
> svc_clean_up_xprts()?

I think you missed the first svc_close_list?:

> >  	svc_close_list(serv, &serv->sv_permsocks, net);
> >+	svc_clean_up_xprts(serv, net);
> >+	svc_close_list(serv, &serv->sv_tempsocks, net);
> >+	svc_clean_up_xprts(serv, net);

The idea is that before we'd like to close all the listeners first, so
that they aren't busy creating more tempsocks while we're trying to
close them.

I overlooked a race, though: if another thread was already handling an
accept for one of the listeners then it might not get closed by that
first svc_clean_up_xprts.

I guess we could do something like:

	delay = 0;

    again:
	numclosed = svc_close_list(serv, &serv->sv_permsocks, net);
	numclosed += svc_close_list(serv, &serv->sv_tempsocks, net);
	if (numclosed) {
		svc_clean_up_xprts(serv, net);
		msleep(delay++);
		goto again;
	}

Seems a little cheesy, but if we don't care much about shutdown
performance in a rare corner case, maybe it's the simplest way out?

--b
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