From nobody Tue Sep 16 21:44:28 2025 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 2F131C3DA7C for ; Wed, 28 Dec 2022 11:06:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232916AbiL1LFc (ORCPT ); Wed, 28 Dec 2022 06:05:32 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48976 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232905AbiL1LFH (ORCPT ); Wed, 28 Dec 2022 06:05:07 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C8F816419 for ; Wed, 28 Dec 2022 03:04:16 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1672225455; 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: content-transfer-encoding:content-transfer-encoding; bh=TUX0sXEcAN0K1aU6BJZ1TjOIBQmXMpyyj+laRKo6qDQ=; b=hUY3pKnNnUZmFVBO+bFII4UDaI9hFc9t/p8HnuEg1w2fo2jZ8T+ylZ972h7fXKDFqVL+Jn AmFK4DOG9Qvt9S0wkqMKAsNr33zFB01kyz0uF/RU8ZMkJvgLtRttPT7zXK700pUZEW8ap1 KvWoa4FMwTF+qbMXja7SKnrkGbLWb+k= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-391-GvfkGOrYPuecNGvFWr0C4g-1; Wed, 28 Dec 2022 06:04:12 -0500 X-MC-Unique: GvfkGOrYPuecNGvFWr0C4g-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id B41C585D066; Wed, 28 Dec 2022 11:04:11 +0000 (UTC) Received: from virtlab701.virt.lab.eng.bos.redhat.com (virtlab701.virt.lab.eng.bos.redhat.com [10.19.152.228]) by smtp.corp.redhat.com (Postfix) with ESMTP id 98C4A40AE1EA; Wed, 28 Dec 2022 11:04:11 +0000 (UTC) From: Paolo Bonzini To: linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: seanjc@google.com Subject: [PATCH] Documentation: kvm: clarify SRCU locking order Date: Wed, 28 Dec 2022 06:04:10 -0500 Message-Id: <20221228110410.1682852-2-pbonzini@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.1 on 10.11.54.1 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" Currently only the locking order of SRCU vs kvm->slots_arch_lock and kvm->slots_lock is documented. Extend this to kvm->lock since Xen emulation got it terribly wrong. Signed-off-by: Paolo Bonzini --- Documentation/virt/kvm/locking.rst | 19 ++++++++++++++----- 1 file changed, 14 insertions(+), 5 deletions(-) diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/lo= cking.rst index 845a561629f1..a3ca76f9be75 100644 --- a/Documentation/virt/kvm/locking.rst +++ b/Documentation/virt/kvm/locking.rst @@ -16,17 +16,26 @@ The acquisition orders for mutexes are as follows: - kvm->slots_lock is taken outside kvm->irq_lock, though acquiring them together is quite rare. =20 -- Unlike kvm->slots_lock, kvm->slots_arch_lock is released before - synchronize_srcu(&kvm->srcu). Therefore kvm->slots_arch_lock - can be taken inside a kvm->srcu read-side critical section, - while kvm->slots_lock cannot. - - kvm->mn_active_invalidate_count ensures that pairs of invalidate_range_start() and invalidate_range_end() callbacks use the same memslots array. kvm->slots_lock and kvm->slots_arch_lock are taken on the waiting side in install_new_memslots, so MMU notifiers must not take either kvm->slots_lock or kvm->slots_arch_lock. =20 +For SRCU: + +- ``synchronize_srcu(&kvm->srcu)`` is called _inside_ + the kvm->slots_lock critical section, therefore kvm->slots_lock + cannot be taken inside a kvm->srcu read-side critical section. + Instead, kvm->slots_arch_lock is released before the call + to ``synchronize_srcu()`` and _can_ be taken inside a + kvm->srcu read-side critical section. + +- kvm->lock is taken inside kvm->srcu, therefore + ``synchronize_srcu(&kvm->srcu)`` cannot be called inside + a kvm->lock critical section. If you cannot delay the + call until after kvm->lock is released, use ``call_srcu``. + On x86: =20 - vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock --=20 2.31.1