[Intel-gfx] [PATCH] drm: Make drm_read() more robust against multithreaded races

[Daniel Vetter]

On Thu, Dec 04, 2014 at 09:03:25PM +0000, Chris Wilson wrote:
> The current implementation of drm_read() faces a number of issues:
> 
> 1. Upon an error, it consumes the event which may lead to the client
> blocking.
> 2. Upon an error, it forgets about events already copied
> 3. If it fails to copy a single event with O_NONBLOCK it falls into a
> infinite loop of reporting EAGAIN.
> 3. There is a race between multiple waiters and blocking reads of the
> events list.
> 
> Here, we inline drm_dequeue_event() into drm_read() so that we can take
> the spinlock around the list walking and event copying, and importantly
> reorder the error handling to avoid the issues above.
> 
> Cc: Takashi Iwai <tiwai at suse.de>
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>

Imo if you go through all the trouble of fixing the corner-cases then we
should also have a testcase to exercise them. Otherwise I expect this to
fall apart again. So a little igt would be great.
-Daniel

> ---
>  drivers/gpu/drm/drm_fops.c | 90 ++++++++++++++++++++++------------------------
>  1 file changed, 43 insertions(+), 47 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_fops.c b/drivers/gpu/drm/drm_fops.c
> index 91e1105f2800..076dd606b580 100644
> --- a/drivers/gpu/drm/drm_fops.c
> +++ b/drivers/gpu/drm/drm_fops.c
> @@ -478,63 +478,59 @@ int drm_release(struct inode *inode, struct file *filp)
>  }
>  EXPORT_SYMBOL(drm_release);
>  
> -static bool
> -drm_dequeue_event(struct drm_file *file_priv,
> -		  size_t total, size_t max, struct drm_pending_event **out)
> +ssize_t drm_read(struct file *filp, char __user *buffer,
> +		 size_t count, loff_t *offset)
>  {
> +	struct drm_file *file_priv = filp->private_data;
>  	struct drm_device *dev = file_priv->minor->dev;
> -	struct drm_pending_event *e;
> -	unsigned long flags;
> -	bool ret = false;
> -
> -	spin_lock_irqsave(&dev->event_lock, flags);
> +	ssize_t ret = 0;
>  
> -	*out = NULL;
> -	if (list_empty(&file_priv->event_list))
> -		goto out;
> -	e = list_first_entry(&file_priv->event_list,
> -			     struct drm_pending_event, link);
> -	if (e->event->length + total > max)
> -		goto out;
> +	if (!access_ok(VERIFY_WRITE, buffer, count))
> +		return -EFAULT;
>  
> -	file_priv->event_space += e->event->length;
> -	list_del(&e->link);
> -	*out = e;
> -	ret = true;
> +	spin_lock_irq(&dev->event_lock);
> +	for (;;) {
> +		if (list_empty(&file_priv->event_list)) {
> +			if (ret)
> +				break;
>  
> -out:
> -	spin_unlock_irqrestore(&dev->event_lock, flags);
> -	return ret;
> -}
> -
> -ssize_t drm_read(struct file *filp, char __user *buffer,
> -		 size_t count, loff_t *offset)
> -{
> -	struct drm_file *file_priv = filp->private_data;
> -	struct drm_pending_event *e;
> -	size_t total;
> -	ssize_t ret;
> +			if (filp->f_flags & O_NONBLOCK) {
> +				ret = -EAGAIN;
> +				break;
> +			}
>  
> -	if ((filp->f_flags & O_NONBLOCK) == 0) {
> -		ret = wait_event_interruptible(file_priv->event_wait,
> -					       !list_empty(&file_priv->event_list));
> -		if (ret < 0)
> -			return ret;
> -	}
> +			spin_unlock_irq(&dev->event_lock);
> +			ret = wait_event_interruptible(file_priv->event_wait,
> +						       !list_empty(&file_priv->event_list));
> +			spin_lock_irq(&dev->event_lock);
> +			if (ret < 0)
> +				break;
> +
> +			ret = 0;
> +		} else {
> +			struct drm_pending_event *e;
> +
> +			e = list_first_entry(&file_priv->event_list,
> +					     struct drm_pending_event, link);
> +			if (e->event->length + ret > count)
> +				break;
> +
> +			if (__copy_to_user_inatomic(buffer + ret,
> +						    e->event, e->event->length)) {
> +				if (ret == 0)
> +					ret = -EFAULT;
> +				break;
> +			}
>  
> -	total = 0;
> -	while (drm_dequeue_event(file_priv, total, count, &e)) {
> -		if (copy_to_user(buffer + total,
> -				 e->event, e->event->length)) {
> -			total = -EFAULT;
> -			break;
> +			file_priv->event_space += e->event->length;
> +			ret += e->event->length;
> +			list_del(&e->link);
> +			e->destroy(e);
>  		}
> -
> -		total += e->event->length;
> -		e->destroy(e);
>  	}
> +	spin_unlock_irq(&dev->event_lock);
>  
> -	return total ?: -EAGAIN;
> +	return ret;
>  }
>  EXPORT_SYMBOL(drm_read);
>  
> -- 
> 2.1.3
> 
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx at lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/intel-gfx

-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch