[PATCH] drm/i915: drop the coditional mutex

[Daniel Vetter]

On Tue, Apr 02, 2013 at 03:30:58PM +0200, Sebastian Andrzej Siewior wrote:
> mutex_is_locked_by() checks the owner of the lock against current. This
> is done by accessing a private member of struct mutex which works on
> mainline but does not on RT.
> I did not figure out, why this "lock-owner-check" and the "lock stealing
> flag" is required. If the lock can not be acquire the lock (because it
> would deadlock) then it can return -1.
> The lock stealing makes actually no sense to me. If
> shrinker_no_lock_stealing is true then the same functions
> (i915_gem_purge(), i915_gem_shrink_all()) are called from
> i915_gem_object_create_mmap_offset() as from i915_gem_inactive_shrink().
> I haven't found a path in which i915_gem_inactive_shrink() is invoked
> from i915_gem_object_create_mmap_offset() that means there is no way
> shrinker_no_lock_stealing is true _and_ the lock is owned by the current
> process.

Every invocation of a memory allocation function can potentially recourse
into the shrinker callbacks. Since we currently protect almost all our gpu
buffer object state with one single lock, and we can also easily hold onto
tons of memory, we can easily OOM without that lock stealing trick.

The patch you're reverting here was added to work around spurious OOM when
trying to allocate sg tables, so this is real.

I fully realize that this is an awful hack and I'll burn on a pretty big
pyre for merging it. I also know that it doesn't really work if someone
else is trying to allocate while another thread is hogging our single
mutex. But without a replacement fix, this is nacked.

The no_lock_stealing hack is probably even worse, we use it to protect
critical section which are touching the gpu lru.

Long term we're working towards that goal (also for a bunch of other
reasons) and aim for more fine-grained locking, so that we can free memory
in smaller chunks. But that's gonna be a lot of work.

Cheers, Daniel


> Since I don't see why the i915 needs this hack while all other user do
> not I recommend to remove this hack and return -1 in case of a dead
> lock. Here is the patch.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
> ---
>  drivers/gpu/drm/i915/i915_drv.h |    1 -
>  drivers/gpu/drm/i915/i915_gem.c |   32 +++-----------------------------
>  2 files changed, 3 insertions(+), 30 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 01769e2..47f28ee 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -724,7 +724,6 @@ struct i915_gem_mm {
>  	struct i915_hw_ppgtt *aliasing_ppgtt;
>  
>  	struct shrinker inactive_shrinker;
> -	bool shrinker_no_lock_stealing;
>  
>  	/**
>  	 * List of objects currently involved in rendering.
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 0e207e6..7949517 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1500,8 +1500,6 @@ static int i915_gem_object_create_mmap_offset(struct drm_i915_gem_object *obj)
>  	if (obj->base.map_list.map)
>  		return 0;
>  
> -	dev_priv->mm.shrinker_no_lock_stealing = true;
> -
>  	ret = drm_gem_create_mmap_offset(&obj->base);
>  	if (ret != -ENOSPC)
>  		goto out;
> @@ -1521,8 +1519,6 @@ static int i915_gem_object_create_mmap_offset(struct drm_i915_gem_object *obj)
>  	i915_gem_shrink_all(dev_priv);
>  	ret = drm_gem_create_mmap_offset(&obj->base);
>  out:
> -	dev_priv->mm.shrinker_no_lock_stealing = false;
> -
>  	return ret;
>  }
>  
> @@ -4368,19 +4364,6 @@ void i915_gem_release(struct drm_device *dev, struct drm_file *file)
>  	spin_unlock(&file_priv->mm.lock);
>  }
>  
> -static bool mutex_is_locked_by(struct mutex *mutex, struct task_struct *task)
> -{
> -	if (!mutex_is_locked(mutex))
> -		return false;
> -
> -#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_MUTEXES)
> -	return mutex->owner == task;
> -#else
> -	/* Since UP may be pre-empted, we cannot assume that we own the lock */
> -	return false;
> -#endif
> -}
> -
>  static int
>  i915_gem_inactive_shrink(struct shrinker *shrinker, struct shrink_control *sc)
>  {
> @@ -4391,18 +4374,10 @@ i915_gem_inactive_shrink(struct shrinker *shrinker, struct shrink_control *sc)
>  	struct drm_device *dev = dev_priv->dev;
>  	struct drm_i915_gem_object *obj;
>  	int nr_to_scan = sc->nr_to_scan;
> -	bool unlock = true;
>  	int cnt;
>  
> -	if (!mutex_trylock(&dev->struct_mutex)) {
> -		if (!mutex_is_locked_by(&dev->struct_mutex, current))
> -			return 0;
> -
> -		if (dev_priv->mm.shrinker_no_lock_stealing)
> -			return 0;
> -
> -		unlock = false;
> -	}
> +	if (!mutex_trylock(&dev->struct_mutex))
> +		return -1;
>  
>  	if (nr_to_scan) {
>  		nr_to_scan -= i915_gem_purge(dev_priv, nr_to_scan);
> @@ -4421,7 +4396,6 @@ i915_gem_inactive_shrink(struct shrinker *shrinker, struct shrink_control *sc)
>  		if (obj->pin_count == 0 && obj->pages_pin_count == 0)
>  			cnt += obj->base.size >> PAGE_SHIFT;
>  
> -	if (unlock)
> -		mutex_unlock(&dev->struct_mutex);
> +	mutex_unlock(&dev->struct_mutex);
>  	return cnt;
>  }
> -- 
> 1.7.6.5
> 

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