From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:44877) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gr24l-0004as-Hw for qemu-devel@nongnu.org; Tue, 05 Feb 2019 09:51:52 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gr21r-0003oA-SV for qemu-devel@nongnu.org; Tue, 05 Feb 2019 09:48:53 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:38896 helo=mx0a-001b2d01.pphosted.com) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gr21o-0003Yz-63 for qemu-devel@nongnu.org; Tue, 05 Feb 2019 09:48:49 -0500 Received: from pps.filterd (m0098413.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x15EcTZF023847 for ; Tue, 5 Feb 2019 09:48:28 -0500 Received: from e11.ny.us.ibm.com (e11.ny.us.ibm.com [129.33.205.201]) by mx0b-001b2d01.pphosted.com with ESMTP id 2qfamdp5qu-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Tue, 05 Feb 2019 09:48:28 -0500 Received: from localhost by e11.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 5 Feb 2019 14:48:27 -0000 References: <20190130132212.7376-1-cohuck@redhat.com> <20190130132212.7376-2-cohuck@redhat.com> <20190130195127.5ff3c849@oc2783563651> <20190131125220.285a4bc8.cohuck@redhat.com> <20190131133455.3097613f@oc2783563651> <20190204163102.33d744d8.cohuck@redhat.com> <20190205125229.56c6e252@oc2783563651> <20190205133540.573902d8.cohuck@redhat.com> From: Eric Farman Date: Tue, 5 Feb 2019 09:48:23 -0500 MIME-Version: 1.0 In-Reply-To: <20190205133540.573902d8.cohuck@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Message-Id: Subject: Re: [Qemu-devel] [PATCH v3 1/6] vfio-ccw: make it safe to access channel programs List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Cornelia Huck , Halil Pasic Cc: linux-s390@vger.kernel.org, Alex Williamson , Pierre Morel , kvm@vger.kernel.org, Farhan Ali , qemu-devel@nongnu.org, qemu-s390x@nongnu.org On 02/05/2019 07:35 AM, Cornelia Huck wrote: > On Tue, 5 Feb 2019 12:52:29 +0100 > Halil Pasic wrote: > >> On Mon, 4 Feb 2019 16:31:02 +0100 >> Cornelia Huck wrote: >> >>> On Thu, 31 Jan 2019 13:34:55 +0100 >>> Halil Pasic wrote: >>> >>>> On Thu, 31 Jan 2019 12:52:20 +0100 >>>> Cornelia Huck wrote: >>>> >>>>> On Wed, 30 Jan 2019 19:51:27 +0100 >>>>> Halil Pasic wrote: >>>>> >>>>>> On Wed, 30 Jan 2019 14:22:07 +0100 >>>>>> Cornelia Huck wrote: >>>>>> >>>>>>> When we get a solicited interrupt, the start function may have >>>>>>> been cleared by a csch, but we still have a channel program >>>>>>> structure allocated. Make it safe to call the cp accessors in >>>>>>> any case, so we can call them unconditionally. >>>>>> >>>>>> I read this like it is supposed to be safe regardless of >>>>>> parallelism and threads. However I don't see any explicit >>>>>> synchronization done for cp->initialized. >>>>>> >>>>>> I've managed to figure out how is that supposed to be safe >>>>>> for the cp_free() (which is probably our main concern) in >>>>>> vfio_ccw_sch_io_todo(), but if fail when it comes to the one >>>>>> in vfio_ccw_mdev_notifier(). >>>>>> >>>>>> Can you explain us how does the synchronization work? >>>>> >>>>> You read that wrong, I don't add synchronization, I just add a check. >>>>> >>>> >>>> Now I'm confused. Does that mean we don't need synchronization for this? >>> >>> If we lack synchronization (that is not provided by the current state >>> machine handling, or the rework here), we should do a patch on top >>> (preferably on top of the whole series, so this does not get even more >>> tangled up.) This is really just about the extra check. >>> >> >> I'm not a huge fan of keeping or introducing races -- it makes things >> difficult to reason about, but I do have some understanging your >> position. > > The only thing I want to avoid is knowingly making things worse than > before, and I don't think this patch does that. > >> >> This patch-series is AFAICT a big improvement over what we have. I would >> like Farhan confirming that it makes these hick-ups when he used to hit >> BUSY with another ssch request disappear. If it does (I hope it does) >> it's definitely a good thing for anybody who wants to use vfio-ccw. > > Yep. There remains a lot to be done, but it's a first step. s/a first step/an excellent first step/ :) Can't speak for Farhan, but this makes things somewhat better for me. I'm still getting some periodic errors, but they happen infrequently enough now that debugging them is frustrating. ;-) - Eric > >> >> Yet I find it difficult to slap my r-b over racy code, or partial >> solutions. In the latter case, when I lack conceptual clarity, I find it >> difficult to tell if we are heading into the right direction, or is what >> we build today going to turn against us tomorrow. Sorry for being a drag. > > As long as we don't introduce bad user space interfaces we have to drag > around forever, I think anything is fair game if we think it's a good > idea at that moment. We can rewrite things if it turned out to be a bad > idea (although I'm not arguing for doing random crap, of course :) >