[03/11] Hack to fix a mutex issue in the DVB layer.

  From: "Steinar H. Gunderson" <sesse@samfundet.no>

dvb_usercopy(), which is called on all ioctls, not only copies data to and from
userspace, but also takes a lock on the file descriptor, which means that only
one ioctl can run at a time. This means that if one thread of mumudvb is busy
trying to get, say, the SNR from the frontend (which can hang due to the issue
above), the CAM thread's ioctl(fd, CA_GET_SLOT_INFO, ...) will hang, even
though it doesn't need to communicate with the hardware at all.  This obviously
requires a better fix, but I don't know the generic DVB layer well enough to
say what it is. Maybe it's some sort of remnant of from when all ioctl()s took
the BKL. Note that on UMP kernels without preemption, mutex_lock is to the best
of my knowledge a no-op, so these delay issues would not show up on non-SMP.

Signed-off-by: Steinar H. Gunderson <sesse@samfundet.no>
---
 drivers/media/dvb/dvb-core/dvbdev.c |    2 --
 1 file changed, 2 deletions(-)
  

Em 01-04-2012 12:53, Steinar H. Gunderson escreveu:
> From: "Steinar H. Gunderson" <sesse@samfundet.no>
> 
> dvb_usercopy(), which is called on all ioctls, not only copies data to and from
> userspace, but also takes a lock on the file descriptor, which means that only
> one ioctl can run at a time. This means that if one thread of mumudvb is busy
> trying to get, say, the SNR from the frontend (which can hang due to the issue
> above), the CAM thread's ioctl(fd, CA_GET_SLOT_INFO, ...) will hang, even
> though it doesn't need to communicate with the hardware at all.  This obviously
> requires a better fix, but I don't know the generic DVB layer well enough to
> say what it is. Maybe it's some sort of remnant of from when all ioctl()s took
> the BKL. Note that on UMP kernels without preemption, mutex_lock is to the best
> of my knowledge a no-op, so these delay issues would not show up on non-SMP.
> 
> Signed-off-by: Steinar H. Gunderson <sesse@samfundet.no>
> ---
>  drivers/media/dvb/dvb-core/dvbdev.c |    2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/drivers/media/dvb/dvb-core/dvbdev.c b/drivers/media/dvb/dvb-core/dvbdev.c
> index 00a6732..e1217f6 100644
> --- a/drivers/media/dvb/dvb-core/dvbdev.c
> +++ b/drivers/media/dvb/dvb-core/dvbdev.c
> @@ -417,10 +417,8 @@ int dvb_usercopy(struct file *file,
>  	}
>  
>  	/* call driver */
> -	mutex_lock(&dvbdev_mutex);
>  	if ((err = func(file, cmd, parg)) == -ENOIOCTLCMD)
>  		err = -EINVAL;
> -	mutex_unlock(&dvbdev_mutex);

As-is, this would be too risky, as it may break random drivers. 

A change like that would require to push down the mutex lock into each caller for
dvb_user_copy:

drivers/media/dvb/dvb-core/dmxdev.c:      return dvb_usercopy(file, cmd, arg, dvb_demux_do_ioctl);
drivers/media/dvb/dvb-core/dmxdev.c:      return dvb_usercopy(file, cmd, arg, dvb_dvr_do_ioctl);
drivers/media/dvb/dvb-core/dvb_ca_en50221.c:      return dvb_usercopy(file, cmd, arg, dvb_ca_en50221_io_do_ioctl);
drivers/media/dvb/dvb-core/dvb_net.c:     return dvb_usercopy(file, cmd, arg, dvb_net_do_ioctl);
drivers/media/dvb/dvb-core/dvbdev.c:      return dvb_usercopy(file, cmd, arg, dvbdev->kernel_ioctl);
drivers/media/dvb/dvb-core/dvbdev.c:int dvb_usercopy(struct file *file,
drivers/media/dvb/dvb-core/dvbdev.h:we simply define out own dvb_usercopy(), which will hopefully become
drivers/media/dvb/dvb-core/dvbdev.h:extern int dvb_usercopy(struct file *file, unsigned int cmd, unsigned long arg,

$ git grep kernel_ioctl drivers/media/dvb/
drivers/media/dvb/dvb-core/dvb_frontend.c:                .kernel_ioctl = dvb_frontend_ioctl
drivers/media/dvb/dvb-core/dvbdev.c:      if (!dvbdev->kernel_ioctl)
drivers/media/dvb/dvb-core/dvbdev.c:      return dvb_usercopy(file, cmd, arg, dvbdev->kernel_ioctl);
drivers/media/dvb/dvb-core/dvbdev.h:      int (*kernel_ioctl)(struct file *file, unsigned int cmd, void *arg);
drivers/media/dvb/firewire/firedtv-ci.c:  .kernel_ioctl   = fdtv_ca_ioctl,
drivers/media/dvb/ttpci/av7110.c: .kernel_ioctl   = dvb_osd_ioctl,
drivers/media/dvb/ttpci/av7110_av.c:      .kernel_ioctl   = dvb_video_ioctl,
drivers/media/dvb/ttpci/av7110_av.c:      .kernel_ioctl   = dvb_audio_ioctl,
drivers/media/dvb/ttpci/av7110_ca.c:      .kernel_ioctl   = dvb_ca_ioctl,

And optimize the code there to avoid uneeded locks.

>  
>  	if (err < 0)
>  		goto out;

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