Re: [PATCH drm-misc-next v5 4/6] drm/gpuvm: track/lock/validate external/evicted objects
From: Boris Brezillon
Date: Tue Oct 03 2023 - 10:21:59 EST
On Tue, 03 Oct 2023 14:25:56 +0200
Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx> wrote:
> > > > +/**
> > > > + * get_next_vm_bo_from_list() - get the next vm_bo element
> > > > + * @__gpuvm: The GPU VM
> > > > + * @__list_name: The name of the list we're iterating on
> > > > + * @__local_list: A pointer to the local list used to store
> > > > already iterated items
> > > > + * @__prev_vm_bo: The previous element we got from
> > > > drm_gpuvm_get_next_cached_vm_bo()
> > > > + *
> > > > + * This helper is here to provide lockless list iteration.
> > > > Lockless as in, the
> > > > + * iterator releases the lock immediately after picking the
> > > > first element from
> > > > + * the list, so list insertion deletion can happen concurrently.
> > > > + *
> > > > + * Elements popped from the original list are kept in a local
> > > > list, so removal
> > > > + * and is_empty checks can still happen while we're iterating
> > > > the list.
> > > > + */
> > > > +#define get_next_vm_bo_from_list(__gpuvm, __list_name,
> > > > __local_list, __prev_vm_bo) \
> > > > + ({
> > > > \
> > > > + struct drm_gpuvm_bo *__vm_bo =
> > > > NULL; \
> > > > +
> > > > \
> > > > + drm_gpuvm_bo_put(__prev_vm_bo);
> > > > \
> > > > +
> > > > \
> > > > + spin_lock(&(__gpuvm)-
> > > > >__list_name.lock); \
> > >
> > > Here we unconditionally take the spinlocks while iterating, and the
> > > main
> > > point of DRM_GPUVM_RESV_PROTECTED was really to avoid that?
> > >
> > >
> > > > + if (!(__gpuvm)-
> > > > >__list_name.local_list) \
> > > > + (__gpuvm)->__list_name.local_list =
> > > > __local_list; \
> > > > + else
> > > > \
> > > > + WARN_ON((__gpuvm)->__list_name.local_list
> > > > != __local_list); \
> > > > +
> > > > \
> > > > + while (!list_empty(&(__gpuvm)->__list_name.list))
> > > > { \
> > > > + __vm_bo = list_first_entry(&(__gpuvm)-
> > > > >__list_name.list, \
> > > > + struct
> > > > drm_gpuvm_bo, \
> > > > +
> > > > list.entry.__list_name); \
> > > > + if (kref_get_unless_zero(&__vm_bo->kref))
> > > > {
> > > And unnecessarily grab a reference in the RESV_PROTECTED case.
> > > > \
> > > > + list_move_tail(&(__vm_bo)-
> > > > >list.entry.__list_name, \
> > > > +
> > > > __local_list); \
> > > > + break;
> > > > \
> > > > + } else
> > > > { \
> > > > + list_del_init(&(__vm_bo)-
> > > > >list.entry.__list_name); \
> > > > + __vm_bo =
> > > > NULL; \
> > > > + }
> > > > \
> > > > + }
> > > > \
> > > > + spin_unlock(&(__gpuvm)-
> > > > >__list_name.lock); \
> > > > +
> > > > \
> > > > + __vm_bo;
> > > > \
> > > > + })
> > >
> > > IMHO this lockless list iteration looks very complex and should be
> > > pretty difficult to maintain while moving forward, also since it
> > > pulls
> > > the gpuvm_bos off the list, list iteration needs to be protected by
> > > an
> > > outer lock anyway.
> >
> > As being partly responsible for this convoluted list iterator, I must
> > say I agree with you. There's so many ways this can go wrong if the
> > user doesn't call it the right way, or doesn't protect concurrent
> > list
> > iterations with a separate lock (luckily, this is a private
> > iterator). I
> > mean, it works, so there's certainly a way to get it right, but gosh,
> > this is so far from the simple API I had hoped for.
> >
> > > Also from what I understand from Boris, the extobj
> > > list would typically not need the fine-grained locking; only the
> > > evict
> > > list?
> >
> > Right, I'm adding the gpuvm_bo to extobj list in the ioctl path, when
> > the GEM and VM resvs are held, and I'm deferring the
> > drm_gpuvm_bo_put()
> > call to a work that's not in the dma-signalling path. This being
> > said,
> > I'm still not comfortable with the
> >
> > gem = drm_gem_object_get(vm_bo->gem);
> > dma_resv_lock(gem->resv);
> > drm_gpuvm_bo_put(vm_bo);
> > dma_resv_unlock(gem->resv);
> > drm_gem_object_put(gem);
> >
> > dance that's needed to avoid a UAF when the gpuvm_bo is the last GEM
> > owner, not to mention that drm_gpuva_unlink() calls
> > drm_gpuvm_bo_put()
> > after making sure the GEM gpuvm_list lock is held, but this lock
> > might
> > differ from the resv lock (custom locking so we can call
> > gpuvm_unlink() in the dma-signalling path). So we now have paths
> > where
> > drm_gpuvm_bo_put() are called with the resv lock held, and others
> > where
> > they are not, and that only works because we're relying on the the
> > fact
> > those drm_gpuvm_bo_put() calls won't make the refcount drop to zero,
> > because the deferred vm_bo_put() work still owns a vm_bo ref.
>
> I'm not sure I follow to 100% here, but in the code snippet above it's
> pretty clear to me that it needs to hold an explicit gem object
> reference when calling dma_resv_unlock(gem->resv). Each time you copy a
> referenced pointer (here from vm_bo->gem to gem) you need to up the
> refcount unless you make sure (by locks or other means) that the source
> of the copy has a strong refcount and stays alive, so that's no weird
> action to me. Could possibly add a drm_gpuvm_bo_get_gem() to access the
> gem member (and that also takes a refcount) for driver users to avoid
> the potential pitfall.
Except this is only needed because of the GEM-resv-must-be-held locking
constraint that was added on vm_bo_put(). I mean, the usual way we do
object un-referencing is by calling _put() and letting the internal
logic undo things when the refcount drops to zero. If the object needs
to be removed from some list, it's normally the responsibility of the
destruction method to lock the list, remove the object and unlock the
list. Now, we have a refcounted object that's referenced by vm_bo, and
whose lock needs to be taken when the destruction happens, which leads
to this weird dance described above, when, in normal situations, we'd
just call drm_gpuvm_bo_put(vm_bo) and let drm_gpuvm do its thing.
>
> >
> > All these tiny details add to the overall complexity of this common
> > layer, and to me, that's not any better than the
> > get_next_vm_bo_from_list() complexity you were complaining about
> > (might
> > be even worth, because this sort of things leak to users).
> >
> > Having an internal lock partly solves that, in that the locking of
> > the
> > extobj list is now entirely orthogonal to the GEM that's being
> > removed
> > from this list, and we can lock/unlock internally without forcing the
> > caller to take weird actions to make sure things don't explode. Don't
> > get me wrong, I get that this locking overhead is not acceptable for
> > Xe, but I feel like we're turning drm_gpuvm into a white elephant
> > that
> > only few people will get right.
>
> I tend to agree, but to me the big complication comes from the async
> (dma signalling path) state updates.
I don't deny updating the VM state from the dma signalling path adds
some amount of complexity, but the fact we're trying to use dma_resv
locks for everything, including protection of internal datasets doesn't
help. Anyway, I think both of us are biased when it comes to judging
which approach adds the most complexity :P.
Also note that, right now, the only thing I'd like to be able to update
from the dma signalling path is the VM mapping tree. Everything else
(drm_gpuva_[un]link(), add/remove extobj), we could do outside this
path:
- for MAP operations, we could call drm_gpuva_link() in the ioctl path
(we'd just need to initialize the drm_gpuva object)
- for MAP operations, we're already calling drm_gpuvm_bo_obtain() from
the ioctl path
- for UNMAP operations, we could add the drm_gpuva_unlink() call to the
VM op cleanup worker
The only problem we'd have is that drm_gpuva_link() needs to be called
inside drm_gpuvm_ops::sm_step_remap() when a remap with next/prev !=
NULL occurs, otherwise we lose track of these mappings.
>
> Let's say for example we have a lower level lock for the gem object's
> gpuvm_bo list. Some drivers grab it from the dma fence signalling path,
> other drivers need to access all vm's of a bo to grab their dma_resv
> locks using a WW transaction. There will be problems, although probably
> solveable.
To me, the gpuvm extobj vm_bo list is just an internal list and has an
internal lock associated. The lock that's protecting the GEM vm_bo list
is a bit different in that the driver gets to decide when a vm_bo is
inserted/removed by calling drm_gpuvm_[un]link(), and can easily make
sure the lock is held when this happens, while the gpuvm internal lists
are kinda transparently updated (for instance, the first caller of
drm_gpuvm_bo_obtain() adds the vm_bo to the extobj and the last vm_bo
owner calling drm_gpuvm_bo_put() removes it from this list, which is
certainly not obvious based on the name of these functions).
If we want to let drivers iterate over the extobj/evict lists, and
assuming they are considered internal lists maintained by the core and
protected with an internal lock, we should indeed provide iteration
helpers that:
1/ make sure all the necessary external locks are held (VM resv, I
guess)
2/ make sure the internal lock is not held during iteration (the sort
of snapshot list trick you're using for the evict list in Xe)
> > > Also it seems that if we are to maintain two modes here, for
> > > reasonably clean code we'd need two separate instances of
> > > get_next_bo_from_list().
> > >
> > > For the !RESV_PROTECTED case, perhaps one would want to consider
> > > the
> > > solution used currently in xe, where the VM maintains two evict
> > > lists.
> > > One protected by a spinlock and one protected by the VM resv. When
> > > the
> > > VM resv is locked to begin list traversal, the spinlock is locked
> > > *once*
> > > and the spinlock-protected list is looped over and copied into the
> > > resv
> > > protected one. For traversal, the resv protected one is used.
> >
> > Oh, so you do have the same sort of trick where you move the entire
> > list to another list, such that you can let other paths update the
> > list
> > while you're iterating your own snapshot. That's interesting...
>
> Yes, it's instead of the "evicted" bool suggested here. I thought the
> latter would be simpler. Although that remains to be seen after all
> use-cases are implemented.
>
> But in general I think the concept of copying from a staging list to
> another with different protection rather than traversing the first list
> and unlocking between items is a good way of solving the locking
> inversion problem with minimal overhead. We use it also for O(1)
> userptr validation.
That's more or less the idea behind get_next_vm_bo_from_list() except
it's dequeuing one element at a time, instead of moving all items at
once. Note that, if you allow concurrent removal protected only by the
spinlock, you still need to take/release this spinlock when iterating
over elements of this snapshot list, because all the remover needs to
remove an element is the element itself, and it doesn't care in which
list it's currently inserted (real or snapshot/staging list), so you'd
be iterating over a moving target if you don't protect the iteration
with the spinlock.