From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:39007) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UZJ5c-0001gh-3n for qemu-devel@nongnu.org; Mon, 06 May 2013 06:56:17 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UZJ5b-0004cy-52 for qemu-devel@nongnu.org; Mon, 06 May 2013 06:56:16 -0400 Received: from goliath.siemens.de ([192.35.17.28]:28740) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UZJ5a-0004cS-SW for qemu-devel@nongnu.org; Mon, 06 May 2013 06:56:15 -0400 Message-ID: <51878C4B.7050700@siemens.com> Date: Mon, 06 May 2013 12:56:11 +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> In-Reply-To: <5187859C.5020305@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 12:27, Paolo Bonzini wrote: > Il 06/05/2013 10:40, Jan Kiszka ha scritto: > >>> >>> [*] The "subscriber link" mechanism allows an LWN.net >>> subscriber to generate a special URL for a >>> subscription-only article. That URL can then be given to >>> others, who will be able to access the article regardless >>> of whether they are subscribed. This feature is made >>> available as a service to LWN subscribers, and in the hope >>> that they will use it to spread the word about their >>> favorite LWN articles. >>> >>>> And memory_region_find should likely always increment a reference >>>> if the target region has an owner. We should convert its users to >>>> properly dereference the region once done with it. >>> >>> Yes. But this is what requires you to have an owner for all regions. >> >> You don't need an owner for regions that are protect by the BQL (the >> majority in the foreseeable future). For those regions, reference >> counting can remain a nop, internally. > > 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. Jan -- Siemens AG, Corporate Technology, CT RTC ITP SDP-DE Corporate Competence Center Embedded Linux