From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:56072) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gqzHZ-0002Cl-E6 for qemu-devel@nongnu.org; Tue, 05 Feb 2019 06:52:54 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gqzHY-0002Qy-Ek for qemu-devel@nongnu.org; Tue, 05 Feb 2019 06:52:53 -0500 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:60354) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gqzHX-0002N1-T6 for qemu-devel@nongnu.org; Tue, 05 Feb 2019 06:52:52 -0500 Received: from pps.filterd (m0098404.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x15Bo48S067453 for ; Tue, 5 Feb 2019 06:52:50 -0500 Received: from e06smtp02.uk.ibm.com (e06smtp02.uk.ibm.com [195.75.94.98]) by mx0a-001b2d01.pphosted.com with ESMTP id 2qf7maet7j-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Tue, 05 Feb 2019 06:52:49 -0500 Received: from localhost by e06smtp02.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 5 Feb 2019 11:52:46 -0000 Date: Tue, 5 Feb 2019 12:52:29 +0100 From: Halil Pasic In-Reply-To: <20190204163102.33d744d8.cohuck@redhat.com> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Message-Id: <20190205125229.56c6e252@oc2783563651> 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 Cc: linux-s390@vger.kernel.org, Eric Farman , Alex Williamson , Pierre Morel , kvm@vger.kernel.org, Farhan Ali , qemu-devel@nongnu.org, qemu-s390x@nongnu.org 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. 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. 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. Regards, Halil