Re: [PATCH] drm/i915: Switch obj->mm.lock lockdep annotations on its head

["Tang, CQ" <cq.tang@intel.com>]

> -----Original Message-----
> From: Daniel Vetter [mailto:daniel.vetter@ffwll.ch]
> Sent: Thursday, August 22, 2019 7:50 AM
> To: Intel Graphics Development <intel-gfx@lists.freedesktop.org>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>; Chris Wilson <chris@chris-
> wilson.co.uk>; Tang, CQ <cq.tang@intel.com>; Ursulin, Tvrtko
> <tvrtko.ursulin@intel.com>; Joonas Lahtinen
> <joonas.lahtinen@linux.intel.com>; Vetter, Daniel <daniel.vetter@intel.com>
> Subject: [PATCH] drm/i915: Switch obj->mm.lock lockdep annotations on its
> head
> 
> The trouble with having a plain nesting flag for locks which do not naturally
> nest (unlike block devices and their partitions, which is the original motivation
> for nesting levels) is that lockdep will never spot a true deadlock if you screw
> up.
> 
> This patch is an attempt at trying better, by highlighting a bit more the actual
> nature of the nesting that's going on. Essentially we have two kinds of
> objects:
> 
> - objects without pages allocated, which cannot be on any lru and are
>   hence inaccessible to the shrinker.
> 
> - objects which have pages allocated, which are on an lru, and which
>   the shrinker can decide to throw out.
> 
> For the former type of object, memory allcoations while holding
> obj->mm.lock are permissible. For the latter they are not. And
> get/put_pages transitions between the two types of objects.
> 
> This is still not entirely fool-proof since the rules might chance.
> But as long as we run such a code ever at runtime lockdep should be able to
> observe the inconsistency and complain (like with any other lockdep class
> that we've split up in multiple classes). But there are a few clear benefits:
> 
> - We can drop the nesting flag parameter from
>   __i915_gem_object_put_pages, because that function by definition is
>   never going allocate memory, and calling it on an object which
>   doesn't have its pages allocated would be a bug.
> 
> - We strictly catch more bugs, since there's not only one place in the
>   entire tree which is annotated with the special class. All the
>   other places that had explicit lockdep nesting annotations we're now
>   going to leave up to lockdep again.
> 
> - Specifically this catches stuff like calling get_pages from
>   put_pages (which isn't really a good idea, if we can call put_pages
>   so could the shrinker). I've seen patches do exactly that.
> 
> Of course I fully expect CI will show me for the fool I am with this one here :-)
> 
> v2: There can only be one (lockdep only has a cache for the first subclass, not
> for deeper ones, and we don't want to make these locks even slower). Still
> separate enums for better documentation.
> 
> Real fix: don forget about phys objs and pin_map(), and fix the shrinker to
> have the right annotations ... silly me.
> 
> v3: Forgot usertptr too ...
> 
> v4: Improve comment for pages_pin_count, drop the IMPORTANT comment
> and instead prime lockdep (Chris).
> 
> v5: Appease checkpatch, no double empty lines (Chris)

Suppose I want to apply the current swapping code on top of this patch, I will have the following locks nested:

Pin_pages(A);-->
mutex_lock_interruptible_nested(&A->mm.lock, I915_MM_GET_PAGES); -->
lock 'struct_mutex'; -->
Find object B to swap out; -->
mutex_lock(&B->mm.lock); -->
create shadow object C; -->
mutex_lock_nested(&C->mm.lock, I915_MM_GET_PAGES);

Is this correct?  Should I lock shadow object C with 'I915_MM_GET_PAGES?   Then A and C are nested with the same subclass.

--CQ


> 
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: "Tang, CQ" <cq.tang@intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_object.c       | 12 +++++++++++-
>  drivers/gpu/drm/i915/gem/i915_gem_object.h       | 16 +++++++++++++---
>  drivers/gpu/drm/i915/gem/i915_gem_object_types.h |  6 +++++-
>  drivers/gpu/drm/i915/gem/i915_gem_pages.c        |  9 ++++-----
>  drivers/gpu/drm/i915/gem/i915_gem_phys.c         |  2 +-
>  drivers/gpu/drm/i915/gem/i915_gem_shrinker.c     |  5 ++---
>  drivers/gpu/drm/i915/gem/i915_gem_userptr.c      |  4 ++--
>  drivers/gpu/drm/i915/gem/selftests/huge_pages.c  | 12 ++++++------
>  8 files changed, 44 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.c
> b/drivers/gpu/drm/i915/gem/i915_gem_object.c
> index d7855dc5a5c5..1bdd7485bc72 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_object.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c
> @@ -22,6 +22,8 @@
>   *
>   */
> 
> +#include <linux/sched/mm.h>
> +
>  #include "display/intel_frontbuffer.h"
>  #include "gt/intel_gt.h"
>  #include "i915_drv.h"
> @@ -51,6 +53,14 @@ void i915_gem_object_init(struct
> drm_i915_gem_object *obj,  {
>  	mutex_init(&obj->mm.lock);
> 
> +	if (IS_ENABLED(CONFIG_LOCKDEP)) {
> +		mutex_lock_nested(&obj->mm.lock,
> I915_MM_GET_PAGES);
> +		fs_reclaim_acquire(GFP_KERNEL);
> +		might_lock(&obj->mm.lock);
> +		fs_reclaim_release(GFP_KERNEL);
> +		mutex_unlock(&obj->mm.lock);
> +	}
> +
>  	spin_lock_init(&obj->vma.lock);
>  	INIT_LIST_HEAD(&obj->vma.list);
> 
> @@ -176,7 +186,7 @@ static void __i915_gem_free_objects(struct
> drm_i915_private *i915,
>  		GEM_BUG_ON(!list_empty(&obj->lut_list));
> 
>  		atomic_set(&obj->mm.pages_pin_count, 0);
> -		__i915_gem_object_put_pages(obj, I915_MM_NORMAL);
> +		__i915_gem_object_put_pages(obj);
>  		GEM_BUG_ON(i915_gem_object_has_pages(obj));
>  		bitmap_free(obj->bit_17);
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.h
> b/drivers/gpu/drm/i915/gem/i915_gem_object.h
> index 5efb9936e05b..a0b1fa8a3224 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_object.h
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.h
> @@ -281,11 +281,21 @@ i915_gem_object_unpin_pages(struct
> drm_i915_gem_object *obj)
> 
>  enum i915_mm_subclass { /* lockdep subclass for obj-
> >mm.lock/struct_mutex */
>  	I915_MM_NORMAL = 0,
> -	I915_MM_SHRINKER /* called "recursively" from direct-reclaim-
> esque */
> +	/*
> +	 * Only used by struct_mutex, when called "recursively" from
> +	 * direct-reclaim-esque. Safe because there is only every one
> +	 * struct_mutex in the entire system. */
> +	I915_MM_SHRINKER = 1,
> +	/*
> +	 * Used for obj->mm.lock when allocating pages. Safe because the
> object
> +	 * isn't yet on any LRU, and therefore the shrinker can't deadlock on
> +	 * it. As soon as the object has pages, obj->mm.lock nests within
> +	 * fs_reclaim.
> +	 */
> +	I915_MM_GET_PAGES = 1,
>  };
> 
> -int __i915_gem_object_put_pages(struct drm_i915_gem_object *obj,
> -				enum i915_mm_subclass subclass);
> +int __i915_gem_object_put_pages(struct drm_i915_gem_object *obj);
>  void i915_gem_object_truncate(struct drm_i915_gem_object *obj);  void
> i915_gem_object_writeback(struct drm_i915_gem_object *obj);
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object_types.h
> b/drivers/gpu/drm/i915/gem/i915_gem_object_types.h
> index ede0eb4218a8..7b7cf711a21a 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_object_types.h
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_object_types.h
> @@ -156,7 +156,11 @@ struct drm_i915_gem_object {
>  	unsigned int pin_global;
> 
>  	struct {
> -		struct mutex lock; /* protects the pages and their use */
> +		/*
> +		 * Protects the pages and their use. Do not use directly, but
> +		 * instead go through the pin/unpin interfaces.
> +		 */
> +		struct mutex lock;
>  		atomic_t pages_pin_count;
> 
>  		struct sg_table *pages;
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_pages.c
> b/drivers/gpu/drm/i915/gem/i915_gem_pages.c
> index 18f0ce0135c1..202526e8910f 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_pages.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_pages.c
> @@ -101,7 +101,7 @@ int __i915_gem_object_get_pages(struct
> drm_i915_gem_object *obj)  {
>  	int err;
> 
> -	err = mutex_lock_interruptible(&obj->mm.lock);
> +	err = mutex_lock_interruptible_nested(&obj->mm.lock,
> +I915_MM_GET_PAGES);
>  	if (err)
>  		return err;
> 
> @@ -179,8 +179,7 @@ __i915_gem_object_unset_pages(struct
> drm_i915_gem_object *obj)
>  	return pages;
>  }
> 
> -int __i915_gem_object_put_pages(struct drm_i915_gem_object *obj,
> -				enum i915_mm_subclass subclass)
> +int __i915_gem_object_put_pages(struct drm_i915_gem_object *obj)
>  {
>  	struct sg_table *pages;
>  	int err;
> @@ -191,7 +190,7 @@ int __i915_gem_object_put_pages(struct
> drm_i915_gem_object *obj,
>  	GEM_BUG_ON(atomic_read(&obj->bind_count));
> 
>  	/* May be called by shrinker from within get_pages() (on another bo)
> */
> -	mutex_lock_nested(&obj->mm.lock, subclass);
> +	mutex_lock(&obj->mm.lock);
>  	if (unlikely(atomic_read(&obj->mm.pages_pin_count))) {
>  		err = -EBUSY;
>  		goto unlock;
> @@ -285,7 +284,7 @@ void *i915_gem_object_pin_map(struct
> drm_i915_gem_object *obj,
>  	if (unlikely(!i915_gem_object_has_struct_page(obj)))
>  		return ERR_PTR(-ENXIO);
> 
> -	err = mutex_lock_interruptible(&obj->mm.lock);
> +	err = mutex_lock_interruptible_nested(&obj->mm.lock,
> +I915_MM_GET_PAGES);
>  	if (err)
>  		return ERR_PTR(err);
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_phys.c
> b/drivers/gpu/drm/i915/gem/i915_gem_phys.c
> index 768356908160..2aea8960f0f1 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_phys.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_phys.c
> @@ -163,7 +163,7 @@ int i915_gem_object_attach_phys(struct
> drm_i915_gem_object *obj, int align)
>  	if (err)
>  		return err;
> 
> -	mutex_lock(&obj->mm.lock);
> +	mutex_lock_nested(&obj->mm.lock, I915_MM_GET_PAGES);
> 
>  	if (obj->mm.madv != I915_MADV_WILLNEED) {
>  		err = -EFAULT;
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shrinker.c
> b/drivers/gpu/drm/i915/gem/i915_gem_shrinker.c
> index edd21d14e64f..0b0d6e27b996 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_shrinker.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_shrinker.c
> @@ -98,7 +98,7 @@ static bool unsafe_drop_pages(struct
> drm_i915_gem_object *obj,
>  		flags = I915_GEM_OBJECT_UNBIND_ACTIVE;
> 
>  	if (i915_gem_object_unbind(obj, flags) == 0)
> -		__i915_gem_object_put_pages(obj, I915_MM_SHRINKER);
> +		__i915_gem_object_put_pages(obj);
> 
>  	return !i915_gem_object_has_pages(obj);  } @@ -254,8 +254,7 @@
> i915_gem_shrink(struct drm_i915_private *i915,
> 
>  			if (unsafe_drop_pages(obj, shrink)) {
>  				/* May arrive from get_pages on another bo
> */
> -				mutex_lock_nested(&obj->mm.lock,
> -						  I915_MM_SHRINKER);
> +				mutex_lock(&obj->mm.lock);
>  				if (!i915_gem_object_has_pages(obj)) {
>  					try_to_writeback(obj, shrink);
>  					count += obj->base.size >>
> PAGE_SHIFT; diff --git a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
> b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
> index 70dc506a5426..f3b3bc7c32cb 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
> @@ -158,7 +158,7 @@ userptr_mn_invalidate_range_start(struct
> mmu_notifier *_mn,
>  		ret = i915_gem_object_unbind(obj,
> 
> I915_GEM_OBJECT_UNBIND_ACTIVE);
>  		if (ret == 0)
> -			ret = __i915_gem_object_put_pages(obj,
> I915_MM_SHRINKER);
> +			ret = __i915_gem_object_put_pages(obj);
>  		i915_gem_object_put(obj);
>  		if (ret)
>  			goto unlock;
> @@ -514,7 +514,7 @@ __i915_gem_userptr_get_pages_worker(struct
> work_struct *_work)
>  		}
>  	}
> 
> -	mutex_lock(&obj->mm.lock);
> +	mutex_lock_nested(&obj->mm.lock, I915_MM_GET_PAGES);
>  	if (obj->userptr.work == &work->work) {
>  		struct sg_table *pages = ERR_PTR(ret);
> 
> diff --git a/drivers/gpu/drm/i915/gem/selftests/huge_pages.c
> b/drivers/gpu/drm/i915/gem/selftests/huge_pages.c
> index 8de83c6d81f5..81af85971856 100644
> --- a/drivers/gpu/drm/i915/gem/selftests/huge_pages.c
> +++ b/drivers/gpu/drm/i915/gem/selftests/huge_pages.c
> @@ -562,7 +562,7 @@ static int igt_mock_ppgtt_misaligned_dma(void *arg)
>  		i915_vma_close(vma);
> 
>  		i915_gem_object_unpin_pages(obj);
> -		__i915_gem_object_put_pages(obj, I915_MM_NORMAL);
> +		__i915_gem_object_put_pages(obj);
>  		i915_gem_object_put(obj);
>  	}
> 
> @@ -590,7 +590,7 @@ static void close_object_list(struct list_head *objects,
> 
>  		list_del(&obj->st_link);
>  		i915_gem_object_unpin_pages(obj);
> -		__i915_gem_object_put_pages(obj, I915_MM_NORMAL);
> +		__i915_gem_object_put_pages(obj);
>  		i915_gem_object_put(obj);
>  	}
>  }
> @@ -860,7 +860,7 @@ static int igt_mock_ppgtt_64K(void *arg)
>  			i915_vma_close(vma);
> 
>  			i915_gem_object_unpin_pages(obj);
> -			__i915_gem_object_put_pages(obj,
> I915_MM_NORMAL);
> +			__i915_gem_object_put_pages(obj);
>  			i915_gem_object_put(obj);
>  		}
>  	}
> @@ -1164,7 +1164,7 @@ static int igt_ppgtt_exhaust_huge(void *arg)
>  			}
> 
>  			i915_gem_object_unpin_pages(obj);
> -			__i915_gem_object_put_pages(obj,
> I915_MM_NORMAL);
> +			__i915_gem_object_put_pages(obj);
>  			i915_gem_object_put(obj);
>  		}
>  	}
> @@ -1226,7 +1226,7 @@ static int igt_ppgtt_internal_huge(void *arg)
>  		}
> 
>  		i915_gem_object_unpin_pages(obj);
> -		__i915_gem_object_put_pages(obj, I915_MM_NORMAL);
> +		__i915_gem_object_put_pages(obj);
>  		i915_gem_object_put(obj);
>  	}
> 
> @@ -1295,7 +1295,7 @@ static int igt_ppgtt_gemfs_huge(void *arg)
>  		}
> 
>  		i915_gem_object_unpin_pages(obj);
> -		__i915_gem_object_put_pages(obj, I915_MM_NORMAL);
> +		__i915_gem_object_put_pages(obj);
>  		i915_gem_object_put(obj);
>  	}
> 
> --
> 2.23.0.rc1

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