From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id BA8BFC433FE for ; Thu, 9 Dec 2021 18:53:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229914AbhLIS4m (ORCPT ); Thu, 9 Dec 2021 13:56:42 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42116 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229838AbhLIS4l (ORCPT ); Thu, 9 Dec 2021 13:56:41 -0500 Received: from bombadil.infradead.org (bombadil.infradead.org [IPv6:2607:7c80:54:e::133]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B0178C061746; Thu, 9 Dec 2021 10:53:07 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20210309; h=MIME-Version:Content-Type:References: In-Reply-To:Date:Cc:To:From:Subject:Message-ID:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=BFrLxGUiIRqxUQhCc8N76rz+mSv72D196MLG7EzwFts=; b=DYoCDW2Axsi5f5VOwddO62QCv/ cQSSQLHpyw41XFgCDJORelDzCdpy3Q5e4pYHl3KH0HHmxI5N6CZBvAgSg7N7bvuoQICv7iYtShmrn FiQJAh4AmW73FzW7NBisqLNNS4DWprUREhNHGNuBmdaXrz71f0nAEpjDIPfSkxnt+iRDIs8bfVM1+ AHp+8+sRRg+X7UKHb6cf8PccoSLSlbyIv/qxCmuCxaKM9hqN3OsbEDbdUJ3mwD/8rw1M73SB0Y09g aNk3yqB8ZV4l86cP9Lxab3dtNwCiNfXhbVs5WJCBu3N70xzZy0JtBBn7EnPByRkJmhu9SIwpWp872 vNW3sM2Q==; Received: from 54-240-197-234.amazon.com ([54.240.197.234] helo=freeip.amazon.com) by bombadil.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1mvOXK-00HHSv-GR; Thu, 09 Dec 2021 18:52:58 +0000 Message-ID: Subject: Re: [PATCH 02/11] rcu: Kill rnp->ofl_seq and use only rcu_state.ofl_lock for exclusion From: David Woodhouse To: paulmck@kernel.org, Neeraj Upadhyay Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Paolo Bonzini , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, rcu@vger.kernel.org, mimoja@mimoja.de, hewenliang4@huawei.com, hushiyuan@huawei.com, luolongjun@huawei.com, hejingxian@huawei.com Date: Thu, 09 Dec 2021 18:52:54 +0000 In-Reply-To: <20211209171835.GD641268@paulmck-ThinkPad-P17-Gen-1> References: <20211209150938.3518-1-dwmw2@infradead.org> <20211209150938.3518-3-dwmw2@infradead.org> <20211209171835.GD641268@paulmck-ThinkPad-P17-Gen-1> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-wGaLQYKPyWhorAKKu7IV" User-Agent: Evolution 3.36.5-0ubuntu1 MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by bombadil.infradead.org. See http://www.infradead.org/rpr.html Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --=-wGaLQYKPyWhorAKKu7IV Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2021-12-09 at 09:18 -0800, Paul E. McKenney wrote: > On Thu, Dec 09, 2021 at 03:09:29PM +0000, David Woodhouse wrote: > > From: David Woodhouse < > > dwmw@amazon.co.uk > > > > >=20 > > If we allow architectures to bring APs online in parallel, then we end > > up requiring rcu_cpu_starting() to be reentrant. But currently, the > > manipulation of rnp->ofl_seq is not thread-safe. > >=20 > > However, rnp->ofl_seq is also fairly much pointless anyway since both > > rcu_cpu_starting() and rcu_report_dead() hold rcu_state.ofl_lock for > > fairly much the whole time that rnp->ofl_seq is set to an odd number > > to indicate that an operation is in progress. > >=20 > > So drop rnp->ofl_seq completely, and use only rcu_state.ofl_lock. > >=20 > > This has a couple of minor complexities: lockdep will complain when we > > take rcu_state.ofl_lock, and currently accepts the 'excuse' of having > > an odd value in rnp->ofl_seq. So switch it to an arch_spinlock_t to > > avoid that false positive complaint. Since we're killing rnp->ofl_seq > > of course that 'excuse' has to be changed too, so make it check for > > arch_spin_is_locked(rcu_state.ofl_lock). > >=20 > > There's no arch_spin_lock_irqsave() so we have to manually save and > > restore local interrupts around the locking. > >=20 > > At Paul's request, make rcu_gp_init not just wait but *exclude* any > > CPU online/offline activity, which was fairly much true already by > > virtue of it holding rcu_state.ofl_lock. >=20 > Looks good! >=20 > Could you please also make the first clause read something like this? >=20 > "At Paul's request based on Neeraj's analysis, ..." >=20 > I am going to pull this into -rcu for more intensive testing of this > code for non-concurrent CPU-online operations (making the above change > to the commit log). At some point, rcutorture needs to learn how to > do concurrent CPU-online operations, but it would be good to bake the > RCU-specific patches for a bit beforehand. >=20 > Depending on timing, you might wish to send this patch with the > rest of this group, so: >=20 > Acked-by: Paul E. McKenney >=20 > If testing goes well and if you don't get it there first, I expect > to push this during the v5.18 merge window. Thanks. I think the best option is probably to let all the cleanups and fixes get merged separately through whatever tree is most appropriate, and then we can just merge that final patch to actually *enable* parallel boot for x86 last of all. I'll address Neeraj's feedback and repost this one. --=-wGaLQYKPyWhorAKKu7IV Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCECow ggUcMIIEBKADAgECAhEA4rtJSHkq7AnpxKUY8ZlYZjANBgkqhkiG9w0BAQsFADCBlzELMAkGA1UE BhMCR0IxGzAZBgNVBAgTEkdyZWF0ZXIgTWFuY2hlc3RlcjEQMA4GA1UEBxMHU2FsZm9yZDEaMBgG A1UEChMRQ09NT0RPIENBIExpbWl0ZWQxPTA7BgNVBAMTNENPTU9ETyBSU0EgQ2xpZW50IEF1dGhl bnRpY2F0aW9uIGFuZCBTZWN1cmUgRW1haWwgQ0EwHhcNMTkwMTAyMDAwMDAwWhcNMjIwMTAxMjM1 OTU5WjAkMSIwIAYJKoZIhvcNAQkBFhNkd213MkBpbmZyYWRlYWQub3JnMIIBIjANBgkqhkiG9w0B AQEFAAOCAQ8AMIIBCgKCAQEAsv3wObLTCbUA7GJqKj9vHGf+Fa+tpkO+ZRVve9EpNsMsfXhvFpb8 RgL8vD+L133wK6csYoDU7zKiAo92FMUWaY1Hy6HqvVr9oevfTV3xhB5rQO1RHJoAfkvhy+wpjo7Q cXuzkOpibq2YurVStHAiGqAOMGMXhcVGqPuGhcVcVzVUjsvEzAV9Po9K2rpZ52FE4rDkpDK1pBK+ uOAyOkgIg/cD8Kugav5tyapydeWMZRJQH1vMQ6OVT24CyAn2yXm2NgTQMS1mpzStP2ioPtTnszIQ 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