Re: [PATCH drm-misc-next v5 4/6] drm/gpuvm: track/lock/validate external/evicted objects

[Danilo Krummrich]

On 10/4/23 17:29, Thomas Hellström wrote:
> On Wed, 2023-10-04 at 14:57 +0200, Danilo Krummrich wrote:
> > On 10/3/23 11:11, Thomas Hellström wrote:
> >
> > <snip>
> >
> > > > > +
> > > > > +/**
> > > > > + * drm_gpuvm_bo_evict() - add / remove a &drm_gpuvm_bo to /
> > > > > from the &drm_gpuvms
> > > > > + * evicted list
> > > > > + * @vm_bo: the &drm_gpuvm_bo to add or remove
> > > > > + * @evict: indicates whether the object is evicted
> > > > > + *
> > > > > + * Adds a &drm_gpuvm_bo to or removes it from the &drm_gpuvms
> > > > > evicted list.
> > > > > + */
> > > > > +void
> > > > > +drm_gpuvm_bo_evict(struct drm_gpuvm_bo *vm_bo, bool evict)
> > > > > +{
> > > > > +    struct drm_gem_object *obj = vm_bo->obj;
> > > > > +
> > > > > +    dma_resv_assert_held(obj->resv);
> > > > > +
> > > > > +    /* Always lock list transactions, even if
> > > > > DRM_GPUVM_RESV_PROTECTED is
> > > > > +     * set. This is required to protect multiple concurrent
> > > > > calls to
> > > > > +     * drm_gpuvm_bo_evict() with BOs with different dma_resv.
> > > > > +     */
> > > >
> > > > This doesn't work. The RESV_PROTECTED case requires the evicted
> > > > flag we discussed before. The list is either protected by the
> > > > spinlock or the resv. Otherwise a list add could race with a list
> > > > removal elsewhere.
> >
> > I think it does unless I miss something, but it might be a bit subtle
> > though.
> >
> > Concurrent drm_gpuvm_bo_evict() are protected by the spinlock.
> > Additionally, when
> > drm_gpuvm_bo_evict() is called we hold the dma-resv of the
> > corresponding GEM object.
> >
> > In drm_gpuvm_validate() I assert that we hold *all* dma-resv, which
> > implies that no
> > one can call drm_gpuvm_bo_evict() on any of the VM's objects and no
> > one can add a new
> > one and directly call drm_gpuvm_bo_evict() on it either.
>
> But translated into how the data (the list in this case) is protected
> it becomes
>
> "Either the spinlock and the bo resv of a single list item OR the bo
> resvs of all bos that can potentially be on the list",
>
> while this is certainly possible to assert, any new / future code that
> manipulates the evict list will probably get this wrong and as a result
> the code becomes pretty fragile. I think drm_gpuvm_bo_destroy() already
> gets it wrong in that it, while holding a single resv, doesn't take the
> spinlock.

That's true and I don't like it either. Unfortunately, with the dma-resv
locking scheme we can't really protect the evict list without the
drm_gpuvm_bo::evicted trick properly.

But as pointed out in my other reply, I'm a bit worried about the
drm_gpuvm_bo::evicted trick being too restrictive, but maybe it's fine
doing it in the RESV_PROTECTED case.

> So I think that needs fixing, and if keeping that protection I think it
> needs to be documented with the list member and ideally an assert. But
> also note that lockdep_assert_held will typically give false true for
> dma_resv locks; as long as the first dma_resv lock locked in a drm_exec
> sequence  remains locked, lockdep thinks *all* dma_resv locks are held.
> (or something along those lines), so the resv lockdep asserts are
> currently pretty useless.
>
> /Thomas
>
>
>
> > > > Thanks,
> > > >
> > > > Thomas