From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: [PATCH v2 6/8] kvm tools: Add rwlock wrapper Date: Mon, 30 May 2011 12:30:20 +0200 Message-ID: <20110530103020.GD17821@elte.hu> References: <1306744247-26051-6-git-send-email-levinsasha928@gmail.com> <20110530084309.GH30513@elte.hu> <1306748069.14564.52.camel@lappy> <1306748796.14564.62.camel@lappy> <20110530095645.GC8461@elte.hu> <1306749934.14564.71.camel@lappy> <20110530101333.GB17821@elte.hu> <1306750963.14564.79.camel@lappy> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Pekka Enberg , kvm@vger.kernel.org, asias.hejun@gmail.com, gorcunov@gmail.com, prasadjoshi124@gmail.com, "Paul E. McKenney" To: Sasha Levin Return-path: Received: from mx2.mail.elte.hu ([157.181.151.9]:51532 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751512Ab1E3Ka2 (ORCPT ); Mon, 30 May 2011 06:30:28 -0400 Content-Disposition: inline In-Reply-To: <1306750963.14564.79.camel@lappy> Sender: kvm-owner@vger.kernel.org List-ID: * Sasha Levin wrote: > On Mon, 2011-05-30 at 12:13 +0200, Ingo Molnar wrote: > > * Sasha Levin wrote: > > > > > On Mon, 2011-05-30 at 11:56 +0200, Ingo Molnar wrote: > > > > * Sasha Levin wrote: > > > > > > > > > I'm just saying that we're limited to as many VCPU threads as we > > > > > can create. br_read_lock() won't do anything on a non-VCPU thread, > > > > > which makes it impossible to test it on non-VCPUs. > > > > > > > > btw., i wondered about that limit - don't we want to fix it? > > > > > > > > I mean, there's no fundamental reason why brlocks should do 'nothing' > > > > in worker threads. In fact it's a subtle breakage waiting AFAICS. > > > > > > Can they do anything useful without locking? I think we should work > > > on integrating an RCU and changing brlocks to use that instead of > > > focusing too much on the current implementation. > > > > What do you mean 'without locking'? If a worker thread uses a > > br_read_lock() then that will be 'locking'. It should map to a real > > read_lock() in the rwlock debug case, etc. > > > I meant without locking anything within br_read_lock(), because we > wanted to keep the read patch lock-free. oh, so it's not recursive. Sane enough - might be worth adding: br_is_read_locked(&lock) and a debug check for that into br_read_lock(): BUG_ON(br_is_read_locked(&lock)); > > > This will also fix that limit you don't like :) > > > > I'd prefer brlocks to more complex solutions in cases where the write > > path is very infrequent! > > > > So we don't want to keep brlocks intentionally crippled. > > Do you see brlock as a global lock that will pause the entire guest > (not just VCPUs - anything except the calling thread)? Yeah, that's how such brlocks work - life has to stop when there's write modifications going on. There should be a mutex around br_write_lock() itself, to make sure two br_write_lock() attempts cannot deadlock each other, but other than that it should be pretty straightforward and robust. And note that such a pause/suspend thing might be helpful to do a *real* host driven suspend feature in the future: stop all vcpus, all worker threads, save state to disk and exit? Thanks, Ingo