From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:44293) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UZJSv-00061W-3b for qemu-devel@nongnu.org; Mon, 06 May 2013 07:20:22 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UZJK6-00012s-9s for qemu-devel@nongnu.org; Mon, 06 May 2013 07:14:34 -0400 Received: from goliath.siemens.de ([192.35.17.28]:23611) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UZJK5-00012D-W5 for qemu-devel@nongnu.org; Mon, 06 May 2013 07:11:14 -0400 Message-ID: <51878FCE.2020907@siemens.com> Date: Mon, 06 May 2013 13:11:10 +0200 From: Jan Kiszka MIME-Version: 1.0 References: <1353808984-22368-1-git-send-email-qemulist@gmail.com> <51829B30.7020308@siemens.com> <51836FA8.2000501@siemens.com> <5184D942.5020506@redhat.com> <5184E601.9030407@web.de> <518764CD.2080602@redhat.com> <51876C86.5040301@siemens.com> <5187859C.5020305@redhat.com> <51878C4B.7050700@siemens.com> <51878CF0.1080009@redhat.com> In-Reply-To: <51878CF0.1080009@redhat.com> Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v7 0/7] push mmio dispatch out of big lock List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: qemu-devel , Liu Ping Fan On 2013-05-06 12:58, Paolo Bonzini wrote: > Il 06/05/2013 12:56, Jan Kiszka ha scritto: >>> The problem is that even if I/O for a region is supposed to happen >>> within the BQL, lookup can happen outside the BQL. Lookup will use the >>> region even if it is just to discard it: >>> >>> VCPU thread (under BQL) device thread >>> -------------------------------------------------------------------------------------- >>> flatview_ref >>> memory_region_find returns d->mr >>> memory_region_ref(d->mr) /* nop */ >>> qdev_free(d) >>> object_unparent(d) >>> unrealize(d) >>> memory_region_del_subregion(d->mr) >>> FlatView updated, d->mr not in the new view >>> >>> flatview_unref >>> memory_region_unref(d->mr) >>> object_unref(d) >>> free(d) >>> if (!d->mr->is_ram) { /* BAD! */ >>> memory_region_unref(d->mr) /* nop */ >>> return error >>> } >>> >>> >>> Here, the memory region is dereferenced *before* we know that it is BQL-free >>> (in fact, exactly to ascertain whether it is BQL-free). >> >> Both flatview update and lookup *plus* locking type evaluation (i.e. >> memory region dereferencing) always happen under the address space lock. >> See Pingfan's patch. > > That's true of address_space_rw/map, but I don't think it holds for > memory_region_find. It has to, or it would be broken: Either it is called on a region that supports reference counting and, thus, increments the counter before returning, or it has to be called with the BQL held. Jan -- Siemens AG, Corporate Technology, CT RTC ITP SDP-DE Corporate Competence Center Embedded Linux