drm/panthor: Lock the VM resv before calling drm_gpuvm_bo_obtain_prealloc()

drm_gpuvm_bo_obtain_prealloc() will call drm_gpuvm_bo_put() on our
pre-allocated BO if the <BO,VM> association exists. Given we
only have one ref on preallocated_vm_bo, drm_gpuvm_bo_destroy() will
be called immediately, and we have to hold the VM resv lock when
calling this function.

Fixes: 647810ec ("drm/panthor: Add the MMU/VM logical block")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240913112722.492144-1-boris.brezillon@collabora.com

drivers/gpu/drm/panthor/panthor_mmu.c

@@ -1251,9 +1251,17 @@ static int panthor_vm_prepare_map_op_ctx(struct panthor_vm_op_ctx *op_ctx,
 		goto err_cleanup;
 	}
 
+	/* drm_gpuvm_bo_obtain_prealloc() will call drm_gpuvm_bo_put() on our
+	 * pre-allocated BO if the <BO,VM> association exists. Given we
+	 * only have one ref on preallocated_vm_bo, drm_gpuvm_bo_destroy() will
+	 * be called immediately, and we have to hold the VM resv lock when
+	 * calling this function.
+	 */
+	dma_resv_lock(panthor_vm_resv(vm), NULL);
 	mutex_lock(&bo->gpuva_list_lock);
 	op_ctx->map.vm_bo = drm_gpuvm_bo_obtain_prealloc(preallocated_vm_bo);
 	mutex_unlock(&bo->gpuva_list_lock);
+	dma_resv_unlock(panthor_vm_resv(vm));
 
 	/* If the a vm_bo for this <VM,BO> combination exists, it already
 	 * retains a pin ref, and we can release the one we took earlier.