From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from perceval.ideasonboard.com ([95.142.166.194]:48799 "EHLO perceval.ideasonboard.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753371Ab3IQWy7 (ORCPT ); Tue, 17 Sep 2013 18:54:59 -0400 From: Laurent Pinchart To: Sakari Ailus Cc: linux-media@vger.kernel.org, hverkuil@xs4all.nl, teemux.tuominen@intel.com Subject: Re: [RFC 1/1] v4l: return POLLERR on V4L2 sub-devices if no events are subscribed Date: Wed, 18 Sep 2013 00:55:01 +0200 Message-ID: <2900512.gZhaQLTEOC@avalon> In-Reply-To: <1379441239-7378-1-git-send-email-sakari.ailus@linux.intel.com> References: <1379441239-7378-1-git-send-email-sakari.ailus@linux.intel.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: linux-media-owner@vger.kernel.org List-ID: Hi Sakari and Teemu, Thank you for the patch. On Tuesday 17 September 2013 21:07:19 Sakari Ailus wrote: > From: Teemu Tuominen > > Add check and return POLLERR from subdev_poll() in case of no events > subscribed and wakeup once the last event subscription is removed. > > This change is essentially done to add possibility to wakeup polling > with concurrent unsubscribe. > > Signed-off-by: Teemu Tuominen > > Move the check after calling poll_wait(). Otherwise it's possible that we go > to sleep without getting notified if the subscription went away between the > two. > > Signed-off-by: Sakari Ailus > Tested-by: Teemu Tuominen > --- > Hi all, > > Poll for events will sleep forever if there are no events subscribed. > Calling poll from an application that has not subscribed any event indeed > sounds silly, but un multi-threaded applications this is not quite as > straightforward. > > Assume the following: an application has two threads where one handles > event subscription and the other handles the events. The first thread > unsubscribes the events and the latter will sleep forever. And do this while > the program intends to quit without an intention to subscribe for any > further events, and there's a deadlock. > > Alternative solutions to handle this are signals (rather a nuisance if the > application happens to be a library instead) or a pipe (2) between the > threads. Pipe is workable, but instead of being a proper solution to the > problem still looks like a workaround instead. pipe() doesn't look like an unproper solution to me, but I'm not against a kernel behaviour change either. Assuming the new behaviour gets accepted, the implementation looks good to me. > This patch fixes the issue on kernel side by waking up the processes > sleeping in poll and returning POLLERR when the last subscribed event is > gone. The behaviour mirrors that of videobuf2 which will return POLLERR if > either streaming is disabled or no buffers are queued. > > Just "waking up the sleeping threads once" is not an option: threads are not > visible to the kernel at this level and just "waking them up" isn't either > since poll will go back to sleep before returning the control back to user > space as long as it would return zero. > > (Thinking about it --- similar change should probably be made to videobuf2 > event poll handling as well.) > > Kind regards, > Sakari > > drivers/media/v4l2-core/v4l2-event.c | 15 +++++++++++++++ > drivers/media/v4l2-core/v4l2-subdev.c | 3 +++ > include/media/v4l2-event.h | 1 + > 3 files changed, 19 insertions(+) > > diff --git a/drivers/media/v4l2-core/v4l2-event.c > b/drivers/media/v4l2-core/v4l2-event.c index 86dcb54..b53897e 100644 > --- a/drivers/media/v4l2-core/v4l2-event.c > +++ b/drivers/media/v4l2-core/v4l2-event.c > @@ -107,6 +107,19 @@ static struct v4l2_subscribed_event > *v4l2_event_subscribed( return NULL; > } > > +bool v4l2_event_has_subscribed(struct v4l2_fh *fh) > +{ > + unsigned long flags; > + bool rval; > + > + spin_lock_irqsave(&fh->vdev->fh_lock, flags); > + rval = !list_empty(&fh->subscribed); > + spin_unlock_irqrestore(&fh->vdev->fh_lock, flags); > + > + return rval; > +} > +EXPORT_SYMBOL(v4l2_event_has_subscribed); > + > static void __v4l2_event_queue_fh(struct v4l2_fh *fh, const struct > v4l2_event *ev, const struct timespec *ts) > { > @@ -299,6 +312,8 @@ int v4l2_event_unsubscribe(struct v4l2_fh *fh, > fh->navailable--; > } > list_del(&sev->list); > + if (list_empty(&fh->subscribed)) > + wake_up_all(&fh->wait); > } > > spin_unlock_irqrestore(&fh->vdev->fh_lock, flags); > diff --git a/drivers/media/v4l2-core/v4l2-subdev.c > b/drivers/media/v4l2-core/v4l2-subdev.c index 996c248..f2aa00f 100644 > --- a/drivers/media/v4l2-core/v4l2-subdev.c > +++ b/drivers/media/v4l2-core/v4l2-subdev.c > @@ -382,6 +382,9 @@ static unsigned int subdev_poll(struct file *file, > poll_table *wait) if (v4l2_event_pending(fh)) > return POLLPRI; > > + if (!v4l2_event_has_subscribed(fh)) > + return POLLERR; > + > return 0; > } > > diff --git a/include/media/v4l2-event.h b/include/media/v4l2-event.h > index be05d01..a9ca2b5 100644 > --- a/include/media/v4l2-event.h > +++ b/include/media/v4l2-event.h > @@ -121,6 +121,7 @@ struct v4l2_subscribed_event { > > int v4l2_event_dequeue(struct v4l2_fh *fh, struct v4l2_event *event, > int nonblocking); > +bool v4l2_event_has_subscribed(struct v4l2_fh *fh); > void v4l2_event_queue(struct video_device *vdev, const struct v4l2_event > *ev); void v4l2_event_queue_fh(struct v4l2_fh *fh, const struct v4l2_event > *ev); int v4l2_event_pending(struct v4l2_fh *fh); -- Regards, Laurent Pinchart