Re: [Intel-gfx] [PATCH v2 12/16] drm/i915: Add i915_vma_unbind_unlocked, and take obj lock for i915_vma_unbind

[Maarten Lankhorst <maarten.lankhorst@linux.intel.com>]

On 09-12-2021 14:05, Matthew Auld wrote:
> On Mon, 29 Nov 2021 at 13:58, Maarten Lankhorst
> <maarten.lankhorst@linux.intel.com> wrote:
>> We want to remove more members of i915_vma, which requires the locking to be
>> held more often.
>>
>> Start requiring gem object lock for i915_vma_unbind, as it's one of the
>> callers that may unpin pages.
>>
>> Some special care is needed when evicting, because the last reference to the
>> object may be held by the VMA, so after __i915_vma_unbind, vma may be garbage,
>> and we need to cache vma->obj before unlocking.
>>
>> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
>> ---
> <snip>
>
>> @@ -129,22 +129,47 @@ void i915_ggtt_suspend_vm(struct i915_address_space *vm)
>>
>>         drm_WARN_ON(&vm->i915->drm, !vm->is_ggtt && !vm->is_dpt);
>>
>> +retry:
>> +       i915_gem_drain_freed_objects(vm->i915);
>> +
>>         mutex_lock(&vm->mutex);
>>
>>         /* Skip rewriting PTE on VMA unbind. */
>>         open = atomic_xchg(&vm->open, 0);
>>
>>         list_for_each_entry_safe(vma, vn, &vm->bound_list, vm_link) {
>> +               struct drm_i915_gem_object *obj = vma->obj;
>> +
>>                 GEM_BUG_ON(!drm_mm_node_allocated(&vma->node));
>> +
>>                 i915_vma_wait_for_bind(vma);
>>
>> -               if (i915_vma_is_pinned(vma))
>> +               if (i915_vma_is_pinned(vma) || !i915_vma_is_bound(vma, I915_VMA_GLOBAL_BIND))
>>                         continue;
>>
>> -               if (!i915_vma_is_bound(vma, I915_VMA_GLOBAL_BIND)) {
>> -                       __i915_vma_evict(vma);
>> -                       drm_mm_remove_node(&vma->node);
>> +               /* unlikely to race when GPU is idle, so no worry about slowpath.. */
>> +               if (!i915_gem_object_trylock(obj, NULL)) {
>> +                       atomic_set(&vm->open, open);
> Does this need a comment about barriers?
Not sure, it's guarded by vm->mutex.
>> +
>> +                       i915_gem_object_get(obj);
> Should this not be kref_get_unless_zero? Assuming the vm->mutex is the
> only thing keeping the object alive here, won't this lead to potential
> uaf/double-free or something? Also should we not plonk this before the
> trylock? Or maybe I'm missing something here?

Normally you're correct, this is normally the case, but we drain freed objects and this path should only be run during s/r, at which point userspace should be dead, GPU idle, and we just drained all freed objects above.

It would be a bug if we still found a dead object, as nothing should be running.

>> +                       mutex_unlock(&vm->mutex);
>> +
>> +                       i915_gem_object_lock(obj, NULL);
>> +                       open = i915_vma_unbind(vma);
>> +                       i915_gem_object_unlock(obj);
>> +
>> +                       GEM_WARN_ON(open);
>> +
>> +                       i915_gem_object_put(obj);
>> +                       goto retry;
>>                 }
>> +
>> +               i915_vma_wait_for_bind(vma);
> We also call wait_for_bind above, is that intentional?

Should be harmless, but first one should probably be removed. :)