From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from mga03.intel.com ([143.182.124.21]:6662 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753128Ab3I3Lpz (ORCPT ); Mon, 30 Sep 2013 07:45:55 -0400 Message-ID: <5249646F.6050706@linux.intel.com> Date: Mon, 30 Sep 2013 14:45:51 +0300 From: Sakari Ailus MIME-Version: 1.0 To: Hans Verkuil CC: linux-media@vger.kernel.org, laurent.pinchart@ideasonboard.com, teemux.tuominen@intel.com Subject: Re: [RFC 1/1] v4l: return POLLERR on V4L2 sub-devices if no events are subscribed References: <1379441239-7378-1-git-send-email-sakari.ailus@linux.intel.com> <52495FAF.9050605@xs4all.nl> In-Reply-To: <52495FAF.9050605@xs4all.nl> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-media-owner@vger.kernel.org List-ID: Hi Hans, Thanks for the review. Hans Verkuil wrote: > Hi Teemu, Sakari, > > I've been thinking about this change and I think it is a reasonable change. > I had some doubts earlier, but after thinking it through I agree with this. > > But it needs a bit more work, see below. > > On 09/17/2013 08:07 PM, 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. >> >> 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.) > > Yes, this should be added here as well. Can you implement that as well? Sure. I thought I'd do that once I get the idea accepted. :-) This will take some time but I'll do that. >> >> 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; >> + > > This needs a bit more work: you should also check that poll() actually is waiting > for exceptions. If not, then also return POLLERR. This has never been checked before, > but it is needed. Good point. I'll fix that. -- Kind regards, Sakari Ailus sakari.ailus@linux.intel.com