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 X-Spam-Level: X-Spam-Status: No, score=-13.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A3209C433DB for ; Tue, 30 Mar 2021 16:56:34 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id EC84361994 for ; Tue, 30 Mar 2021 16:56:33 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org EC84361994 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:55590 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lRHfN-0007jk-0J for qemu-devel@archiver.kernel.org; Tue, 30 Mar 2021 12:56:33 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:59470) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lRHe1-00076v-RV for qemu-devel@nongnu.org; Tue, 30 Mar 2021 12:55:09 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:50979) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lRHdz-0001dz-E3 for qemu-devel@nongnu.org; Tue, 30 Mar 2021 12:55:09 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1617123306; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=guHRS33eXArM2dG5UXbhC9Lz+W6VS6idO1v1fgWCt5s=; b=ed/Kh6jUInArVua5Uc9wfxx0Vzt4dPiS27Wl/ACKZttZIx9mv71Mm6OmC/RMRn0pH3RH5z 3DUmP97roAic3jUr6QoMcRxKROqglrBzYNrYGzEd4H7wEuGpDOOOY20AB537VUhPmASQlh 3tLxcoZGhNEPibl0y6FN0SLDAoFaTtk= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-487-pJ_tUOWiOOaamCTTzC18eg-1; Tue, 30 Mar 2021 12:55:03 -0400 X-MC-Unique: pJ_tUOWiOOaamCTTzC18eg-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id D143118C8C00; Tue, 30 Mar 2021 16:55:02 +0000 (UTC) Received: from localhost (ovpn-115-22.ams2.redhat.com [10.36.115.22]) by smtp.corp.redhat.com (Postfix) with ESMTP id 63C5118966; Tue, 30 Mar 2021 16:55:02 +0000 (UTC) Date: Tue, 30 Mar 2021 17:55:01 +0100 From: Stefan Hajnoczi To: Paolo Bonzini Subject: Re: [PATCH v6 4/6] coroutine-lock: Reimplement CoRwlock to fix downgrade bug Message-ID: References: <20210325112941.365238-1-pbonzini@redhat.com> <20210325112941.365238-5-pbonzini@redhat.com> MIME-Version: 1.0 In-Reply-To: <20210325112941.365238-5-pbonzini@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=stefanha@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="kyUPYghkFZogJgWo" Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: david.edmondson@oracle.com, qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" --kyUPYghkFZogJgWo Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Thu, Mar 25, 2021 at 12:29:39PM +0100, Paolo Bonzini wrote: > An invariant of the current rwlock is that if multiple coroutines hold a > reader lock, all must be runnable. The unlock implementation relies on > this, choosing to wake a single coroutine when the final read lock > holder exits the critical section, assuming that it will wake a > coroutine attempting to acquire a write lock. >=20 > The downgrade implementation violates this assumption by creating a > read lock owning coroutine that is exclusively runnable - any other > coroutines that are waiting to acquire a read lock are *not* made > runnable when the write lock holder converts its ownership to read > only. >=20 > More in general, the old implementation had lots of other fairness bugs. > The root cause of the bugs was that CoQueue would wake up readers even > if there were pending writers, and would wake up writers even if there > were readers. In that case, the coroutine would go back to sleep *at > the end* of the CoQueue, losing its place at the head of the line. >=20 > To fix this, keep the queue of waiters explicitly in the CoRwlock > instead of using CoQueue, and store for each whether it is a > potential reader or a writer. This way, downgrade can look at the > first queued coroutines and wake it only if it is a reader, causing > all other readers in line to be released in turn. >=20 > Reported-by: David Edmondson > Reviewed-by: David Edmondson > Signed-off-by: Paolo Bonzini > --- > v3->v4: clean up the code and fix upgrade logic. Fix upgrade comment too= . >=20 > include/qemu/coroutine.h | 17 +++-- > util/qemu-coroutine-lock.c | 148 ++++++++++++++++++++++++------------- > 2 files changed, 106 insertions(+), 59 deletions(-) Reviewed-by: Stefan Hajnoczi --kyUPYghkFZogJgWo Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmBjV+UACgkQnKSrs4Gr c8gSJwgAmNn/ADv7r2BJTrnEaKZS671vGLHgE/Tvxcykq0Z5WwQbflIKGD9zmpT5 S5ENWxGDnfl5jOnOWoq3HqxJv5ukm0k8KnhhrBmY2DdIGF5EKqcX0h1oQkHH/BwA qBoQ2S/XM7SjwVyetCygwZOx3nMPcBx5ZSSGTh6hAEgJZAqvT0RHcUWf8GQygsoc JeQTPK1p3JnEx/0J3XQbKiFmj3rsiGd70XTL3xMMEF7+CNbEu2NDDTZX7fZAdyav Eq8W4juraIK0aPzYpn6xqqqt2F9Np2Gz5YCk7/2vwXoaB/a6OU8r7upo3ZK/8eQH lf410IS+JF3I1gB4bV+RLgUXDm3c3g== =/WGz -----END PGP SIGNATURE----- --kyUPYghkFZogJgWo--