Re: [PATCH] drm/amdgpu: Fix potential race processing vm->freed

[Rob Clark]

On Mon, Feb 6, 2023 at 2:15 AM Christian König <christian.koenig@xxxxxxx> wrote:
>
> Am 03.02.23 um 19:10 schrieb Rob Clark:
> > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> >
> > If userspace calls the AMDGPU_CS ioctl from multiple threads, because
> > the vm is global to the drm_file, you can end up with multiple threads
> > racing in amdgpu_vm_clear_freed().  So the freed list should be
> > protected with the status_lock, similar to other vm lists.
>
> Well this is nonsense. To process the freed list the VM root PD lock
> must be held anyway.
>
> If we have a call path where this isn't true then we have a major bug at
> a different place here.

I'm not super familiar w/ the amdgpu cs parser stuff, but the only
thing that I'm seeing that protects things is the bo_list_mutex and it
isn't clear to me that this is 1:1 with the vm (it looks like it is
not).

(I cc'd you on the bug report, jfyi)

BR,
-R

>
> Regards,
> Christian.
>
> >
> > Fixes: d38ceaf99ed0 ("drm/amdgpu: add core driver (v4)")
> > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
> > ---
> >   drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c | 33 ++++++++++++++++++++++----
> >   1 file changed, 29 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
> > index b9441ab457ea..aeed7bc1512f 100644
> > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
> > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
> > @@ -1240,10 +1240,19 @@ int amdgpu_vm_clear_freed(struct amdgpu_device *adev,
> >       struct amdgpu_bo_va_mapping *mapping;
> >       uint64_t init_pte_value = 0;
> >       struct dma_fence *f = NULL;
> > +     struct list_head freed;
> >       int r;
> >
> > -     while (!list_empty(&vm->freed)) {
> > -             mapping = list_first_entry(&vm->freed,
> > +     /*
> > +      * Move the contents of the VM's freed list to a local list
> > +      * that we can iterate without racing against other threads:
> > +      */
> > +     spin_lock(&vm->status_lock);
> > +     list_replace_init(&vm->freed, &freed);
> > +     spin_unlock(&vm->status_lock);
> > +
> > +     while (!list_empty(&freed)) {
> > +             mapping = list_first_entry(&freed,
> >                       struct amdgpu_bo_va_mapping, list);
> >               list_del(&mapping->list);
> >
> > @@ -1258,6 +1267,15 @@ int amdgpu_vm_clear_freed(struct amdgpu_device *adev,
> >               amdgpu_vm_free_mapping(adev, vm, mapping, f);
> >               if (r) {
> >                       dma_fence_put(f);
> > +
> > +                     /*
> > +                      * Move any unprocessed mappings back to the freed
> > +                      * list:
> > +                      */
> > +                     spin_lock(&vm->status_lock);
> > +                     list_splice_tail(&freed, &vm->freed);
> > +                     spin_unlock(&vm->status_lock);
> > +
> >                       return r;
> >               }
> >       }
> > @@ -1583,11 +1601,14 @@ int amdgpu_vm_bo_unmap(struct amdgpu_device *adev,
> >       mapping->bo_va = NULL;
> >       trace_amdgpu_vm_bo_unmap(bo_va, mapping);
> >
> > -     if (valid)
> > +     if (valid) {
> > +             spin_lock(&vm->status_lock);
> >               list_add(&mapping->list, &vm->freed);
> > -     else
> > +             spin_unlock(&vm->status_lock);
> > +     } else {
> >               amdgpu_vm_free_mapping(adev, vm, mapping,
> >                                      bo_va->last_pt_update);
> > +     }
> >
> >       return 0;
> >   }
> > @@ -1671,7 +1692,9 @@ int amdgpu_vm_bo_clear_mappings(struct amdgpu_device *adev,
> >                   tmp->last = eaddr;
> >
> >               tmp->bo_va = NULL;
> > +             spin_lock(&vm->status_lock);
> >               list_add(&tmp->list, &vm->freed);
> > +             spin_unlock(&vm->status_lock);
> >               trace_amdgpu_vm_bo_unmap(NULL, tmp);
> >       }
> >
> > @@ -1788,7 +1811,9 @@ void amdgpu_vm_bo_del(struct amdgpu_device *adev,
> >               amdgpu_vm_it_remove(mapping, &vm->va);
> >               mapping->bo_va = NULL;
> >               trace_amdgpu_vm_bo_unmap(bo_va, mapping);
> > +             spin_lock(&vm->status_lock);
> >               list_add(&mapping->list, &vm->freed);
> > +             spin_unlock(&vm->status_lock);
> >       }
> >       list_for_each_entry_safe(mapping, next, &bo_va->invalids, list) {
> >               list_del(&mapping->list);
>