Re: [PATCH 1/3] drm/i915: Use RPM as the barrier for controlling user mmap access

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Oct 10, 2016 at 10:58:40PM +0100, Chris Wilson wrote:
> We can remove the false coupling between RPM and struct mutex by the
> observation that we can use the RPM wakeref as the barrier around user
> mmap access. That is as we tear down the user's PTE atomically from
> within rpm suspend and then to fault in new PTE requires the rpm
> wakeref, means that no user access is possible through those PTE without
> RPM being awake. Having made that observation, we can then remove the
> presumption of having to take rpm outside of struct_mutex and so allow
> fine grained acquisition of a wakeref around hw access rather than
> having to remember to acquire the wakeref early on.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Imre Deak <imre.deak@intel.com>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/i915_debugfs.c    | 22 ----------------------
>  drivers/gpu/drm/i915/i915_drv.c        | 19 -------------------
>  drivers/gpu/drm/i915/i915_gem.c        | 29 ++++++++++++++++++-----------
>  drivers/gpu/drm/i915/i915_gem_gtt.c    | 17 +++++++++++++----
>  drivers/gpu/drm/i915/i915_gem_tiling.c |  4 ----
>  drivers/gpu/drm/i915/intel_drv.h       |  6 ++++++
>  drivers/gpu/drm/i915/intel_uncore.c    |  6 +++---
>  7 files changed, 40 insertions(+), 63 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
> index 20689f1cd719..793b1816f700 100644
> --- a/drivers/gpu/drm/i915/i915_debugfs.c
> +++ b/drivers/gpu/drm/i915/i915_debugfs.c
> @@ -1396,14 +1396,9 @@ static int i915_hangcheck_info(struct seq_file *m, void *unused)
>  static int ironlake_drpc_info(struct seq_file *m)
>  {
>  	struct drm_i915_private *dev_priv = node_to_i915(m->private);
> -	struct drm_device *dev = &dev_priv->drm;
>  	u32 rgvmodectl, rstdbyctl;
>  	u16 crstandvid;
> -	int ret;
>  
> -	ret = mutex_lock_interruptible(&dev->struct_mutex);
> -	if (ret)
> -		return ret;
>  	intel_runtime_pm_get(dev_priv);
>  
>  	rgvmodectl = I915_READ(MEMMODECTL);
> @@ -1411,7 +1406,6 @@ static int ironlake_drpc_info(struct seq_file *m)
>  	crstandvid = I915_READ16(CRSTANDVID);
>  
>  	intel_runtime_pm_put(dev_priv);
> -	mutex_unlock(&dev->struct_mutex);
>  
>  	seq_printf(m, "HD boost: %s\n", yesno(rgvmodectl & MEMMODE_BOOST_EN));
>  	seq_printf(m, "Boost freq: %d\n",
> @@ -2089,12 +2083,7 @@ static const char *swizzle_string(unsigned swizzle)
>  static int i915_swizzle_info(struct seq_file *m, void *data)
>  {
>  	struct drm_i915_private *dev_priv = node_to_i915(m->private);
> -	struct drm_device *dev = &dev_priv->drm;
> -	int ret;
>  
> -	ret = mutex_lock_interruptible(&dev->struct_mutex);
> -	if (ret)
> -		return ret;
>  	intel_runtime_pm_get(dev_priv);
>  
>  	seq_printf(m, "bit6 swizzle for X-tiling = %s\n",
> @@ -2134,7 +2123,6 @@ static int i915_swizzle_info(struct seq_file *m, void *data)
>  		seq_puts(m, "L-shaped memory detected\n");
>  
>  	intel_runtime_pm_put(dev_priv);
> -	mutex_unlock(&dev->struct_mutex);
>  
>  	return 0;
>  }
> @@ -4793,13 +4781,9 @@ i915_wedged_set(void *data, u64 val)
>  	if (i915_reset_in_progress(&dev_priv->gpu_error))
>  		return -EAGAIN;
>  
> -	intel_runtime_pm_get(dev_priv);
> -
>  	i915_handle_error(dev_priv, val,
>  			  "Manually setting wedged to %llu", val);
>  
> -	intel_runtime_pm_put(dev_priv);
> -
>  	return 0;
>  }
>  
> @@ -5034,22 +5018,16 @@ static int
>  i915_cache_sharing_get(void *data, u64 *val)
>  {
>  	struct drm_i915_private *dev_priv = data;
> -	struct drm_device *dev = &dev_priv->drm;
>  	u32 snpcr;
> -	int ret;
>  
>  	if (!(IS_GEN6(dev_priv) || IS_GEN7(dev_priv)))
>  		return -ENODEV;
>  
> -	ret = mutex_lock_interruptible(&dev->struct_mutex);
> -	if (ret)
> -		return ret;
>  	intel_runtime_pm_get(dev_priv);
>  
>  	snpcr = I915_READ(GEN6_MBCUNIT_SNPCR);
>  
>  	intel_runtime_pm_put(dev_priv);
> -	mutex_unlock(&dev->struct_mutex);
>  
>  	*val = (snpcr & GEN6_MBC_SNPCR_MASK) >> GEN6_MBC_SNPCR_SHIFT;
>  
> diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
> index 89d322215c84..31eee32fcf6d 100644
> --- a/drivers/gpu/drm/i915/i915_drv.c
> +++ b/drivers/gpu/drm/i915/i915_drv.c
> @@ -2313,24 +2313,6 @@ static int intel_runtime_suspend(struct device *kdev)
>  
>  	DRM_DEBUG_KMS("Suspending device\n");
>  
> -	/*
> -	 * We could deadlock here in case another thread holding struct_mutex
> -	 * calls RPM suspend concurrently, since the RPM suspend will wait
> -	 * first for this RPM suspend to finish. In this case the concurrent
> -	 * RPM resume will be followed by its RPM suspend counterpart. Still
> -	 * for consistency return -EAGAIN, which will reschedule this suspend.
> -	 */
> -	if (!mutex_trylock(&dev->struct_mutex)) {
> -		DRM_DEBUG_KMS("device lock contention, deffering suspend\n");
> -		/*
> -		 * Bump the expiration timestamp, otherwise the suspend won't
> -		 * be rescheduled.
> -		 */
> -		pm_runtime_mark_last_busy(kdev);
> -
> -		return -EAGAIN;
> -	}
> -
>  	disable_rpm_wakeref_asserts(dev_priv);
>  
>  	/*
> @@ -2338,7 +2320,6 @@ static int intel_runtime_suspend(struct device *kdev)
>  	 * an RPM reference.
>  	 */
>  	i915_gem_release_all_mmaps(dev_priv);
> -	mutex_unlock(&dev->struct_mutex);

i915_gem_release_all_mmaps is walking the bound_list, which afaict is
protected by dev->struct_mutex still. I guess a separate lock for that
list would be ok, but I don't see it anywhere.

Requiring that we hold a full rpm reference before touching this list
would be another approach, but imo that defeats the point of this patch of
decoupling rpm and struct_mutex locking.

Otherwise makes sense I think, same for patch2.
-Daniel

>  
>  	intel_guc_suspend(dev);
>  
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index a89a88922448..ee920b0ec258 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1469,7 +1469,9 @@ i915_gem_pwrite_ioctl(struct drm_device *dev, void *data,
>  	 */
>  	if (!i915_gem_object_has_struct_page(obj) ||
>  	    cpu_write_needs_clflush(obj)) {
> +		intel_runtime_pm_get(dev_priv);
>  		ret = i915_gem_gtt_pwrite_fast(dev_priv, obj, args, file);
> +		intel_runtime_pm_put(dev_priv);
>  		/* Note that the gtt paths might fail with non-page-backed user
>  		 * pointers (e.g. gtt mappings when moving data between
>  		 * textures). Fallback to the shmem path in that case. */
> @@ -1922,9 +1924,13 @@ i915_gem_release_mmap(struct drm_i915_gem_object *obj)
>  	/* Serialisation between user GTT access and our code depends upon
>  	 * revoking the CPU's PTE whilst the mutex is held. The next user
>  	 * pagefault then has to wait until we release the mutex.
> +	 *
> +	 * Note that RPM complicates somewhat by adding an additional
> +	 * requirement that operations to the GGTT be made holding the RPM
> +	 * wakeref. This in turns allow us to release the mmap from within
> +	 * the RPM suspend code ignoring the struct_mutex serialisation in
> +	 * lieu of the RPM barriers.
>  	 */
> -	lockdep_assert_held(&obj->base.dev->struct_mutex);
> -
>  	if (!obj->fault_mappable)
>  		return;
>  
> @@ -1948,6 +1954,11 @@ i915_gem_release_all_mmaps(struct drm_i915_private *dev_priv)
>  {
>  	struct drm_i915_gem_object *obj;
>  
> +	/* This should only be called by RPM as we require the bound_list
> +	 * to be protected by the RPM barriers and not struct_mutex.
> +	 * We check that we are holding the wakeref whenever we manipulate
> +	 * the dev_priv->mm.bound_list (via assert_rpm_release_all_mmaps).
> +	 */
>  	list_for_each_entry(obj, &dev_priv->mm.bound_list, global_list)
>  		i915_gem_release_mmap(obj);
>  }
> @@ -2911,9 +2922,11 @@ int i915_vma_unbind(struct i915_vma *vma)
>  
>  	/* Since the unbound list is global, only move to that list if
>  	 * no more VMAs exist. */
> -	if (--obj->bind_count == 0)
> +	if (--obj->bind_count == 0) {
> +		assert_rpm_release_all_mmaps(to_i915(obj->base.dev));
>  		list_move_tail(&obj->global_list,
>  			       &to_i915(obj->base.dev)->mm.unbound_list);
> +	}
>  
>  	/* And finally now the object is completely decoupled from this vma,
>  	 * we can drop its hold on the backing storage and allow it to be
> @@ -3099,6 +3112,7 @@ search_free:
>  	}
>  	GEM_BUG_ON(!i915_gem_valid_gtt_space(vma, obj->cache_level));
>  
> +	assert_rpm_release_all_mmaps(dev_priv);
>  	list_move_tail(&obj->global_list, &dev_priv->mm.bound_list);
>  	list_move_tail(&vma->vm_link, &vma->vm->inactive_list);
>  	obj->bind_count++;
> @@ -3446,7 +3460,6 @@ int i915_gem_get_caching_ioctl(struct drm_device *dev, void *data,
>  int i915_gem_set_caching_ioctl(struct drm_device *dev, void *data,
>  			       struct drm_file *file)
>  {
> -	struct drm_i915_private *dev_priv = to_i915(dev);
>  	struct drm_i915_gem_caching *args = data;
>  	struct drm_i915_gem_object *obj;
>  	enum i915_cache_level level;
> @@ -3475,11 +3488,9 @@ int i915_gem_set_caching_ioctl(struct drm_device *dev, void *data,
>  		return -EINVAL;
>  	}
>  
> -	intel_runtime_pm_get(dev_priv);
> -
>  	ret = i915_mutex_lock_interruptible(dev);
>  	if (ret)
> -		goto rpm_put;
> +		return ret;
>  
>  	obj = i915_gem_object_lookup(file, args->handle);
>  	if (!obj) {
> @@ -3488,13 +3499,9 @@ int i915_gem_set_caching_ioctl(struct drm_device *dev, void *data,
>  	}
>  
>  	ret = i915_gem_object_set_cache_level(obj, level);
> -
>  	i915_gem_object_put(obj);
>  unlock:
>  	mutex_unlock(&dev->struct_mutex);
> -rpm_put:
> -	intel_runtime_pm_put(dev_priv);
> -
>  	return ret;
>  }
>  
> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
> index 0bb4232f66bc..e1109d723060 100644
> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
> @@ -2596,6 +2596,7 @@ static int ggtt_bind_vma(struct i915_vma *vma,
>  			 enum i915_cache_level cache_level,
>  			 u32 flags)
>  {
> +	struct drm_i915_private *i915 = to_i915(vma->vm->dev);
>  	struct drm_i915_gem_object *obj = vma->obj;
>  	u32 pte_flags = 0;
>  	int ret;
> @@ -2608,8 +2609,10 @@ static int ggtt_bind_vma(struct i915_vma *vma,
>  	if (obj->gt_ro)
>  		pte_flags |= PTE_READ_ONLY;
>  
> +	intel_runtime_pm_get(i915);
>  	vma->vm->insert_entries(vma->vm, vma->pages, vma->node.start,
>  				cache_level, pte_flags);
> +	intel_runtime_pm_put(i915);
>  
>  	/*
>  	 * Without aliasing PPGTT there's no difference between
> @@ -2625,6 +2628,7 @@ static int aliasing_gtt_bind_vma(struct i915_vma *vma,
>  				 enum i915_cache_level cache_level,
>  				 u32 flags)
>  {
> +	struct drm_i915_private *i915 = to_i915(vma->vm->dev);
>  	u32 pte_flags;
>  	int ret;
>  
> @@ -2639,14 +2643,15 @@ static int aliasing_gtt_bind_vma(struct i915_vma *vma,
>  
>  
>  	if (flags & I915_VMA_GLOBAL_BIND) {
> +		intel_runtime_pm_get(i915);
>  		vma->vm->insert_entries(vma->vm,
>  					vma->pages, vma->node.start,
>  					cache_level, pte_flags);
> +		intel_runtime_pm_put(i915);
>  	}
>  
>  	if (flags & I915_VMA_LOCAL_BIND) {
> -		struct i915_hw_ppgtt *appgtt =
> -			to_i915(vma->vm->dev)->mm.aliasing_ppgtt;
> +		struct i915_hw_ppgtt *appgtt = i915->mm.aliasing_ppgtt;
>  		appgtt->base.insert_entries(&appgtt->base,
>  					    vma->pages, vma->node.start,
>  					    cache_level, pte_flags);
> @@ -2657,13 +2662,17 @@ static int aliasing_gtt_bind_vma(struct i915_vma *vma,
>  
>  static void ggtt_unbind_vma(struct i915_vma *vma)
>  {
> -	struct i915_hw_ppgtt *appgtt = to_i915(vma->vm->dev)->mm.aliasing_ppgtt;
> +	struct drm_i915_private *i915 = to_i915(vma->vm->dev);
> +	struct i915_hw_ppgtt *appgtt = i915->mm.aliasing_ppgtt;
>  	const u64 size = min(vma->size, vma->node.size);
>  
> -	if (vma->flags & I915_VMA_GLOBAL_BIND)
> +	if (vma->flags & I915_VMA_GLOBAL_BIND) {
> +		intel_runtime_pm_get(i915);
>  		vma->vm->clear_range(vma->vm,
>  				     vma->node.start, size,
>  				     true);
> +		intel_runtime_pm_put(i915);
> +	}
>  
>  	if (vma->flags & I915_VMA_LOCAL_BIND && appgtt)
>  		appgtt->base.clear_range(&appgtt->base,
> diff --git a/drivers/gpu/drm/i915/i915_gem_tiling.c b/drivers/gpu/drm/i915/i915_gem_tiling.c
> index a14b1e3d4c78..08f796a4f5f6 100644
> --- a/drivers/gpu/drm/i915/i915_gem_tiling.c
> +++ b/drivers/gpu/drm/i915/i915_gem_tiling.c
> @@ -204,8 +204,6 @@ i915_gem_set_tiling(struct drm_device *dev, void *data,
>  		return -EINVAL;
>  	}
>  
> -	intel_runtime_pm_get(dev_priv);
> -
>  	mutex_lock(&dev->struct_mutex);
>  	if (obj->pin_display || obj->framebuffer_references) {
>  		err = -EBUSY;
> @@ -301,8 +299,6 @@ err:
>  	i915_gem_object_put(obj);
>  	mutex_unlock(&dev->struct_mutex);
>  
> -	intel_runtime_pm_put(dev_priv);
> -
>  	return err;
>  }
>  
> diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
> index f48e79ae2ac6..9b0a2fa7984b 100644
> --- a/drivers/gpu/drm/i915/intel_drv.h
> +++ b/drivers/gpu/drm/i915/intel_drv.h
> @@ -1648,6 +1648,12 @@ assert_rpm_wakelock_held(struct drm_i915_private *dev_priv)
>  		DRM_DEBUG_DRIVER("RPM wakelock ref not held during HW access");
>  }
>  
> +static inline void
> +assert_rpm_release_all_mmaps(struct drm_i915_private *dev_priv)
> +{
> +	assert_rpm_wakelock_held(dev_priv);
> +}
> +
>  static inline int
>  assert_rpm_atomic_begin(struct drm_i915_private *dev_priv)
>  {
> diff --git a/drivers/gpu/drm/i915/intel_uncore.c b/drivers/gpu/drm/i915/intel_uncore.c
> index e2b188dcf908..e0c3bd941e38 100644
> --- a/drivers/gpu/drm/i915/intel_uncore.c
> +++ b/drivers/gpu/drm/i915/intel_uncore.c
> @@ -1364,7 +1364,7 @@ int i915_reg_read_ioctl(struct drm_device *dev,
>  	struct register_whitelist const *entry = whitelist;
>  	unsigned size;
>  	i915_reg_t offset_ldw, offset_udw;
> -	int i, ret = 0;
> +	int i, ret;
>  
>  	for (i = 0; i < ARRAY_SIZE(whitelist); i++, entry++) {
>  		if (i915_mmio_reg_offset(entry->offset_ldw) == (reg->offset & -entry->size) &&
> @@ -1386,6 +1386,7 @@ int i915_reg_read_ioctl(struct drm_device *dev,
>  
>  	intel_runtime_pm_get(dev_priv);
>  
> +	ret = 0;
>  	switch (size) {
>  	case 8 | 1:
>  		reg->val = I915_READ64_2x32(offset_ldw, offset_udw);
> @@ -1404,10 +1405,9 @@ int i915_reg_read_ioctl(struct drm_device *dev,
>  		break;
>  	default:
>  		ret = -EINVAL;
> -		goto out;
> +		break;
>  	}
>  
> -out:
>  	intel_runtime_pm_put(dev_priv);
>  	return ret;
>  }
> -- 
> 2.9.3
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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