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

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

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.

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
> > > 
> > > 
> > 
>