[Intel-gfx] [PATCH] RFC: drm/i915: Switch obj->mm.lock lockdep annotations on its head
Daniel Vetter
daniel.vetter at ffwll.ch
Fri Aug 16 07:28:41 UTC 2019
On Thu, Aug 15, 2019 at 9:35 PM Tang, CQ <cq.tang at intel.com> wrote:
>
>
>
> > -----Original Message-----
> > From: Intel-gfx [mailto:intel-gfx-bounces at lists.freedesktop.org] On Behalf
> > Of Daniel Vetter
> > Sent: Wednesday, August 14, 2019 12:25 PM
> > To: Intel Graphics Development <intel-gfx at lists.freedesktop.org>
> > Cc: Daniel Vetter <daniel.vetter at ffwll.ch>; Vetter, Daniel
> > <daniel.vetter at intel.com>
> > Subject: [Intel-gfx] [PATCH] RFC: 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 ...
>
> I eventually looked this patch. My question is on the shrinking calling stack:
>
> Pin_pages(A)-->get_page(A)-->lock(objA->mm.lock, I915_MM_GET_PAGES)-->i915_gem_shrink()-->
> Lock(struct_mutex)-->put_pages(B)-->lock(objB->mm.lock)....
>
> objA is locked with: mutex_lock_interruptible_nested(&obj->mm.lock, I915_MM_GET_PAGES);
>
> objB is locked with: mutex_lock(&obj->mm.lock);
> My understanding is that objB locking is equivalently to:
> mutex_lock_nested(&obj->mm.lock, I915_MM_NORMAL);
>
> so you lock subclass=2 first on A, then lock subclass=0 next B, the reverse order.
> Doesn't this cause a lockdep warning?
So mutex_lock_nested really is misnamed, it should be called
mutex_lock_subclass. There's no guarantees/promises about a certain
nesting, just that lockdep should use the 1st/2nd/... subclass of the
lockdep class the lock uses (there's a limit of 8 total, including the
main/default subclass). From lockdep's pov a subclass works exactly
like allocating a new lockdep key. But with the difference that you
can change it at runtime, i.e. when an object makes a state
transition, you can change the subclass you're using. Like this patch
tries to do here.
But ofc like everytime you're creating a new lockdep class, you need
to spend a few words on why it's not hiding real issues from lockdep.
-Daniel
> --CQ
>
>
> >
> > Cc: Chris Wilson <chris at chris-wilson.co.uk>
> > 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 | 2 +-
> > drivers/gpu/drm/i915/gem/i915_gem_object.h | 16 +++++++++++++---
> > drivers/gpu/drm/i915/gem/i915_gem_object_types.h | 10 +++++++++-
> > 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, 38 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 3929c3a6b281..a1a835539e09 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_object.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c
> > @@ -191,7 +191,7 @@ 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 3714cf234d64..5ce511ca7fa8 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 d474c6ac4100..1ea3c3c96a5a 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_object_types.h
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_object_types.h
> > @@ -157,7 +157,15 @@ 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.
> > + *
> > + * IMPORTANT: It is not allowed to allocate memory while
> > holding
> > + * this lock, because the shrinker might recurse on it, except
> > + * when there are no pages allocated yet and the object isn't
> > + * visible on any LRU.
> > + */
> > + 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 102fd7a23d3d..209925be8a76 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_phys.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_phys.c
> > @@ -156,7 +156,7 @@ 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 6cbd4a668c9a..df586035c33e 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);
> > }
> > }
> > @@ -1268,7 +1268,7 @@ 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);
> > }
> > }
> > @@ -1330,7 +1330,7 @@ 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);
> > }
> >
> > @@ -1399,7 +1399,7 @@ 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);
> > }
> >
> > --
> > 2.22.0
> >
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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