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[72.137.123.47]) by smtp.gmail.com with ESMTPSA id w1sm5343645qtk.31.2019.12.16.07.07.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 16 Dec 2019 07:07:54 -0800 (PST) Date: Mon, 16 Dec 2019 10:07:54 -0500 From: Peter Xu To: "Michael S. Tsirkin" Cc: Paolo Bonzini , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Sean Christopherson , "Dr . David Alan Gilbert" , Vitaly Kuznetsov Subject: Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking Message-ID: <20191216150754.GC83861@xz-x1> References: <20191129213505.18472-1-peterx@redhat.com> <20191129213505.18472-5-peterx@redhat.com> <20191211063830-mutt-send-email-mst@kernel.org> <20191211205952.GA5091@xz-x1> <20191211172713-mutt-send-email-mst@kernel.org> <46ceb88c-0ddd-0d9a-7128-3aa5a7d9d233@redhat.com> <20191215173302.GB83861@xz-x1> <20191216044619-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20191216044619-mutt-send-email-mst@kernel.org> User-Agent: Mutt/1.12.1 (2019-06-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 16, 2019 at 04:47:36AM -0500, Michael S. Tsirkin wrote: > On Sun, Dec 15, 2019 at 12:33:02PM -0500, Peter Xu wrote: > > On Thu, Dec 12, 2019 at 01:08:14AM +0100, Paolo Bonzini wrote: > > > >>> What depends on what here? Looks suspicious ... > > > >> > > > >> Hmm, I think maybe it can be removed because the entry pointer > > > >> reference below should be an ordering constraint already? > > > > > > entry->xxx depends on ring->reset_index. > > > > Yes that's true, but... > > > > entry = &ring->dirty_gfns[ring->reset_index & (ring->size - 1)]; > > /* barrier? */ > > next_slot = READ_ONCE(entry->slot); > > next_offset = READ_ONCE(entry->offset); > > > > ... I think entry->xxx depends on entry first, then entry depends on > > reset_index. So it seems fine because all things have a dependency? > > Is reset_index changed from another thread then? > If yes then you want to read reset_index with READ_ONCE. > That includes a dependency barrier. There're a few readers, but only this function will change it (kvm_dirty_ring_reset). Thanks, -- Peter Xu