Quoting Daniel Vetter (2019-08-16 19:23:36)
> 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).
>
> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: "Tang, CQ" <cq.tang@xxxxxxxxx>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> ---
> drivers/gpu/drm/i915/gem/i915_gem_object.c | 13 ++++++++++++-
> 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, 45 insertions(+), 22 deletions(-)
static inline int __must_check
i915_gem_object_pin_pages(struct drm_i915_gem_object *obj)
{
might_lock(&obj->mm.lock);
if (atomic_inc_not_zero(&obj->mm.pages_pin_count))
return 0;
return __i915_gem_object_get_pages(obj);
}
is now testing the wrong lock class.
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.c b/drivers/gpu/drm/i915/gem/i915_gem_object.c
> index 3929c3a6b281..d01258b175f5 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"
> @@ -61,6 +63,15 @@ 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);
> + }
This is very powerful and sells a lot of churn.
-Chris
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