Return-Path: Received: from fieldses.org ([173.255.197.46]:46496 "EHLO fieldses.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752069AbbJLUUo (ORCPT ); Mon, 12 Oct 2015 16:20:44 -0400 Date: Mon, 12 Oct 2015 16:20:44 -0400 From: "J. Bruce Fields" To: Jeff Layton Cc: linux-nfs@vger.kernel.org Subject: Re: [PATCH] nfsd: use a multithreaded workqueue for nfsd4_callbacks Message-ID: <20151012202044.GL28755@fieldses.org> References: <1443875882-12089-1-git-send-email-jeff.layton@primarydata.com> <20151009212459.GF8188@fieldses.org> <20151009173923.1ca297a4@synchrony.poochiereds.net> <20151012181117.GF28755@fieldses.org> <20151012161421.1468a79b@synchrony.poochiereds.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20151012161421.1468a79b@synchrony.poochiereds.net> Sender: linux-nfs-owner@vger.kernel.org List-ID: On Mon, Oct 12, 2015 at 04:14:21PM -0400, Jeff Layton wrote: > On Mon, 12 Oct 2015 14:11:17 -0400 > "J. Bruce Fields" wrote: > > > On Fri, Oct 09, 2015 at 05:39:23PM -0400, Jeff Layton wrote: > > > On Fri, 9 Oct 2015 17:24:59 -0400 > > > "J. Bruce Fields" wrote: > > > > > > > On Sat, Oct 03, 2015 at 08:38:02AM -0400, Jeff Layton wrote: > > > > > I don't see any need to order callbacks with respect to one another. > > > > > > > > I thought the code in nfsd4_process_cb_update really depended on this. > > > > The locking it has is against nfsd threads, it probably assumes it's > > > > only run from a cb thread and that it's the only one running at a time. > > > > > > > > But I haven't reviewed it lately. > > > > > > > > --b. > > > > > > > > > > Yikes -- ok. That's not at all obvious. That should prob be documented > > > if so. > > > > Yes, my bad. > > > > > Yeah, ok...I guess you could end up with multiple threads racing to > > > tear down the cb_client and xprt and create a new one, and it looks > > > like the cl_cb_client and cl_cred pointers could get clobbered by new > > > ones in that case. > > > > > > Shouldn't be too hard to fix protecting those pointers with the > > > cl_lock. That said, I prob won't be able to spend time on it right now. > > > You can go ahead and drop that patch and I'll resend if/when I get > > > around to fixing that. > > > > What's the problem that this fixes exactly? > > > > Unnecessary serialization of callbacks? > > Not that that's necessarily a problem, but with the newer workqueue > implementation there is more overhead in running a single threaded > workqueue since it implies a rescuer thread and has to do extra work > to ensure that the jobs are run in sequential order. > > If that serialization is not actually needed, then it's better to move > it to a multithreaded workqueue. We'd still be serializing a lot of the same code, just using locks instead, wouldn't we? --b.