Re: [PATCH 1/2] [media] media-device: Fix mutex handling code for ioctl

[Mauro Carvalho Chehab <mchehab@osg.samsung.com>]

Em Tue, 29 Mar 2016 00:58:37 +0300
Sakari Ailus <sakari.ailus@iki.fi> escreveu:

> Hi Mauro,
> 
> Please see the comments below.
> 
> On Mon, Mar 28, 2016 at 03:11:03PM -0300, Mauro Carvalho Chehab wrote:
> > Remove two nested mutex left-overs at find_entity and make sure
> > that the code won't suffer race conditions if the device is
> > being removed while ioctl is being handled nor the topology changes,
> > by protecting all ioctls with a mutex at media_device_ioctl().
> > 
> > As reported by Laurent, commit c38077d39c7e ("[media] media-device:
> > get rid of the spinlock") introduced a deadlock in the
> > MEDIA_IOC_ENUM_LINKS ioctl handler:
> > 
> > [ 2760.127749] INFO: task media-ctl:954 blocked for more than 120 seconds.
> > [ 2760.131867]       Not tainted 4.5.0+ #357
> > [ 2760.134622] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > [ 2760.139310] media-ctl       D ffffffc000086bcc     0   954    671 0x00000001
> > [ 2760.143618] Call trace:
> > [ 2760.145601] [<ffffffc000086bcc>] __switch_to+0x90/0xa4
> > [ 2760.148941] [<ffffffc0004e6ef0>] __schedule+0x188/0x5b0
> > [ 2760.152309] [<ffffffc0004e7354>] schedule+0x3c/0xa0
> > [ 2760.155495] [<ffffffc0004e7768>] schedule_preempt_disabled+0x20/0x38
> > [ 2760.159423] [<ffffffc0004e8d28>] __mutex_lock_slowpath+0xc4/0x148
> > [ 2760.163217] [<ffffffc0004e8df0>] mutex_lock+0x44/0x5c
> > [ 2760.166483] [<ffffffc0003e87d4>] find_entity+0x2c/0xac
> > [ 2760.169773] [<ffffffc0003e8d34>] __media_device_enum_links+0x20/0x1dc
> > [ 2760.173711] [<ffffffc0003e9718>] media_device_ioctl+0x214/0x33c
> > [ 2760.177384] [<ffffffc0003e9eec>] media_ioctl+0x24/0x3c
> > [ 2760.180671] [<ffffffc0001bee64>] do_vfs_ioctl+0xac/0x758
> > [ 2760.184026] [<ffffffc0001bf594>] SyS_ioctl+0x84/0x98
> > [ 2760.187196] [<ffffffc000085d30>] el0_svc_naked+0x24/0x28
> > 
> > That's because find_entity() holds the graph_mutex, but both
> > MEDIA_IOC_ENUM_LINKS and MEDIA_IOC_SETUP_LINK logic also take
> > the mutex.
> > 
> > Reported-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
> > Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
> > ---
> >  drivers/media/media-device.c | 12 ++----------
> >  1 file changed, 2 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/media/media-device.c b/drivers/media/media-device.c
> > index 7bfb2b24f644..6cfa890af7b4 100644
> > --- a/drivers/media/media-device.c
> > +++ b/drivers/media/media-device.c
> > @@ -90,8 +90,6 @@ static struct media_entity *find_entity(struct media_device *mdev, u32 id)
> >  
> >  	id &= ~MEDIA_ENT_ID_FLAG_NEXT;
> >  
> > -	mutex_lock(&mdev->graph_mutex);
> > -
> >  	media_device_for_each_entity(entity, mdev) {
> >  		if (((media_entity_id(entity) == id) && !next) ||
> >  		    ((media_entity_id(entity) > id) && next)) {
> > @@ -100,8 +98,6 @@ static struct media_entity *find_entity(struct media_device *mdev, u32 id)
> >  		}
> >  	}
> >  
> > -	mutex_unlock(&mdev->graph_mutex);  
> 
> find_entity() will return the entity inside the loop if it's found,
> releasing the mutex.

Actually, I folded such hunk on patch 2 by mistake.

> 
> > -
> >  	return NULL;
> >  }
> >  
> > @@ -431,6 +427,7 @@ static long media_device_ioctl(struct file *filp, unsigned int cmd,
> >  	struct media_device *dev = devnode->media_dev;
> >  	long ret;
> >  
> > +	mutex_lock(&dev->graph_mutex);
> >  	switch (cmd) {
> >  	case MEDIA_IOC_DEVICE_INFO:
> >  		ret = media_device_get_info(dev,
> > @@ -443,29 +440,24 @@ static long media_device_ioctl(struct file *filp, unsigned int cmd,
> >  		break;
> >  
> >  	case MEDIA_IOC_ENUM_LINKS:
> > -		mutex_lock(&dev->graph_mutex);
> >  		ret = media_device_enum_links(dev,
> >  				(struct media_links_enum __user *)arg);
> > -		mutex_unlock(&dev->graph_mutex);
> >  		break;
> >  
> >  	case MEDIA_IOC_SETUP_LINK:
> > -		mutex_lock(&dev->graph_mutex);
> >  		ret = media_device_setup_link(dev,
> >  				(struct media_link_desc __user *)arg);
> > -		mutex_unlock(&dev->graph_mutex);
> >  		break;
> >  
> >  	case MEDIA_IOC_G_TOPOLOGY:
> > -		mutex_lock(&dev->graph_mutex);
> >  		ret = media_device_get_topology(dev,
> >  				(struct media_v2_topology __user *)arg);
> > -		mutex_unlock(&dev->graph_mutex);
> >  		break;
> >  
> >  	default:
> >  		ret = -ENOIOCTLCMD;
> >  	}
> > +	mutex_unlock(&dev->graph_mutex);
> >  
> >  	return ret;
> >  }  
> 


-- 
Thanks,
Mauro