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

Fri Aug 16 22:02:41 UTC 2019

On Fri, Aug 16, 2019 at 8:46 PM Chris Wilson <chris at chris-wilson.co.uk> wrote:
>
> 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 at chris-wilson.co.uk>
> > Cc: "Tang, CQ" <cq.tang at intel.com>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > ---
> >  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.

Unfortunately there's no might_lock_nested.

But then, this is the best kind of wrong, because of the nesting we have:

obj->mm.lock#I915_MM_GET_PAGES -> fs_reclaim -> obj->mm.lock

So the might_lock we have actually checks for way more than just the
"more correct" annotation. I think I'll just add the above as a
comment and leave the code as-is. Thoughts?

> > 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.

Yeah that was the idea here. Plus I hope it's the easier to understand
the annotations and lock nesting rules for obj->mm.lock this way - I
freaked out quite a bit about the current one until you convinced me
(which took it's sweet time) that it's all fine. Maybe explicitly
annotating get_pages and it's special rule will help others (I can't
play guinea pig twice unfortunately, so we can't test that theory).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch