From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from mail-qc0-f175.google.com ([209.85.216.175]:44715 "EHLO mail-qc0-f175.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751545AbbCJOJ2 (ORCPT ); Tue, 10 Mar 2015 10:09:28 -0400 Received: by qcwr17 with SMTP id r17so1977766qcw.11 for ; Tue, 10 Mar 2015 07:09:27 -0700 (PDT) Message-ID: <54FEFA83.2080205@vanguardiasur.com.ar> Date: Tue, 10 Mar 2015 11:06:59 -0300 From: Ezequiel Garcia MIME-Version: 1.0 To: Hans Verkuil , linux-media@vger.kernel.org, mchehab@osg.samsung.com, hans.verkuil@cisco.com Subject: Re: em38xx locking question References: <54FEEF38.6060506@vanguardiasur.com.ar> <54FEF0E9.9070804@xs4all.nl> <54FEF1D1.3000909@vanguardiasur.com.ar> <54FEF5B4.1060209@xs4all.nl> In-Reply-To: <54FEF5B4.1060209@xs4all.nl> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-media-owner@vger.kernel.org List-ID: On 03/10/2015 10:46 AM, Hans Verkuil wrote: > On 03/10/2015 02:29 PM, Ezequiel Garcia wrote: >> >> >> On 03/10/2015 10:26 AM, Hans Verkuil wrote: >>> On 03/10/2015 02:18 PM, Ezequiel Garcia wrote: >>>> Mauro, >>>> >>>> Function drivers/media/usb/em28xx/em28xx-video.c:get_next_buf >>>> (copy pasted below for reference) does not take the list spinlock, >>>> yet it modifies the list. Is that correct? >>> >>> That looks wrong to me. You really need spinlocks here. >>> >> >> OK, second question then. Is there any way to guarantee the URBs irq handler >> is *not* running, when vb2_ops are called (e.g. stop_streaming)? > > That depends on the op. But stop_streaming is the op that is supposed to > turn off the streaming (and thus the irq), so it depends on the order > of how things are done in that function. > Ah, right. As long as you kill the urbs before you try to access the current buffer, everything is OK. >> Otherwise, given stop_streaming will return the current buffer to vb2 >> (dev->usb_ctl.vid_buf), I believe that will race against the irq handler, >> which is processing it. >> >> It seems that's currently racy as well. > > Hmm, the stop_streaming code looks fine at first sight, but I think there > is a race if you start streaming both video and vbi, and then stop streaming > one of the two. I think the code might keep calling get_next_buf() in that > case, even if for that stream the streaming was stopped. > > This is a problem anyway: get_next_buf() should do this check at the beginning: > > if (!vb2_start_streaming_called(vb2_queue)) > return NULL; > > to prevent it from using buffer before start_streaming was actually called. > I'd say get_next_buf() is called only in the URB complete handler path, and hence only after start_streaming. However, maybe there's a subtle issue here: URB complete handler can be called _while_ start_streaming is still running. -- Ezequiel Garcia, VanguardiaSur www.vanguardiasur.com.ar