drm/etnaviv: check for reaped mapping in etnaviv_iommu_unmap_gem

commit e168c25 upstream.

When the mapping is already reaped the unmap must be a no-op, as we
would otherwise try to remove the mapping twice, corrupting the involved
data structures.

Cc: stable@vger.kernel.org # 5.4
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Tested-by: Guido Günther <agx@sigxcpu.org>
Acked-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/etnaviv/etnaviv_mmu.c

@@ -286,6 +286,12 @@ void etnaviv_iommu_unmap_gem(struct etnaviv_iommu_context *context,
 
 	mutex_lock(&context->lock);
 
+	/* Bail if the mapping has been reaped by another thread */
+	if (!mapping->context) {
+		mutex_unlock(&context->lock);
+		return;
+	}
+
 	/* If the vram node is on the mm, unmap and remove the node */
 	if (mapping->vram_node.mm == &context->mm)
 		etnaviv_iommu_remove_mapping(context, mapping);