Re: [PATCH] DVB: dvb_frontend: fix delayed thread exit

Devin Heitmueller
2011-12-10T01:59:36Z

On Fri, Dec 9, 2011 at 8:37 PM, Andreas Oberritter <obi@linuxtv.org> wrote:
> On 10.12.2011 00:43, Mauro Carvalho Chehab wrote:
>> On 09-12-2011 21:37, Mauro Carvalho Chehab wrote:
>>> On 09-12-2011 20:33, Devin Heitmueller wrote:
>>>> On Fri, Dec 9, 2011 at 5:11 PM, Mauro Carvalho Chehab
>>>> <mchehab@redhat.com> wrote:
>>>>>> Could someone explain reason for that?
>>>>>
>>>>>
>>>>> I dunno, but I think this needs to be fixed, at least when the frontend
>>>>> is opened with O_NONBLOCK.
>>>>
>>>> Are you doing the drx-k firmware load on dvb_init()? That could
>>>> easily take 4 seconds.
>>>
>>> No. The firmware were opened previously.
>>
>> Maybe the delay is due to this part of dvb_frontend.c:
>>
>> static int dvb_mfe_wait_time = 5;
>> ...
>>                         int mferetry = (dvb_mfe_wait_time << 1);
>>
>>                         mutex_unlock (&adapter->mfe_lock);
>>                         while (mferetry-- && (mfedev->users != -1 ||
>>                                         mfepriv->thread != NULL)) {
>>                                 if(msleep_interruptible(500)) {
>>                                         if(signal_pending(current))
>>                                                 return -EINTR;
>>                                 }
>>                         }
>
> I haven't looked at the mfe code, but in case it's waiting for the
> frontend thread to exit, there's a problem that causes the thread
> not to exit immediately. Here's a patch that's been sitting in my
> queue for a while:
>
> ---
>
> Signed-off-by: Andreas Oberritter <obi@linuxtv.org>
>
> diff --git a/linux/drivers/media/dvb/dvb-core/dvb_frontend.c b/linux/drivers/media/dvb/dvb-core/dvb_frontend.c
> index 7784d74..6823c2b 100644
> --- a/linux/drivers/media/dvb/dvb-core/dvb_frontend.c   2011-09-07 12:32:24.000000000 +0200
> +++ a/linux/drivers/media/dvb/dvb-core/dvb_frontend.c   2011-09-13 15:55:48.865742791 +0200
> @@ -514,7 +514,7 @@
>                return 1;
>
>        if (fepriv->dvbdev->writers == 1)
> -               if (time_after(jiffies, fepriv->release_jiffies +
> +               if (time_after_eq(jiffies, fepriv->release_jiffies +
>                                  dvb_shutdown_timeout * HZ))
>                        return 1;
>
> @@ -2070,12 +2070,15 @@
>
>        dprintk ("%s\n", __func__);
>
> -       if ((file->f_flags & O_ACCMODE) != O_RDONLY)
> +       if ((file->f_flags & O_ACCMODE) != O_RDONLY) {
>                fepriv->release_jiffies = jiffies;
> +               mb();
> +       }
>
>        ret = dvb_generic_release (inode, file);
>
>        if (dvbdev->users == -1) {
> +               wake_up(&fepriv->wait_queue);
>                if (fepriv->exit != DVB_FE_NO_EXIT) {
>                        fops_put(file->f_op);
>                        file->f_op = NULL;

This patch needs to have a much better explanation of exactly what it
does and what problem it solves. We have a history of race conditions
in dvb_frontend.c, and it's patches like this with virtually no
details just makes it worse.

I'm not arguing the actual merits of the code change - it *may* be
correct. But without the appropriate background there is no real way
of knowing...

Mauro, this patch should be NACK'd and resubmitted with a detailed
explanation of the current behavior, what the problem is, and how the
code changes proposed solve that problem.

Devin

--
Devin J. Heitmueller - Kernel Labs