[Intel-xe] [PATCH] drm/xe: Decrement fault mode counts in xe_vm_close_and_put

[Niranjana Vishwanathapura]

On Tue, Mar 21, 2023 at 09:08:15AM -0700, Matthew Brost wrote:
>Rather waiting for the VM to be destroyed (all refs to VM go to zero),
>drop the fault mode counts when the VM is closed in xe_vm_close_and_put.
>This avoids a window where user space can create a faulting VM, close
>it, and a subsequent creation of a non-faulting VM fails.
>
>v2 (Lucas): Drop VLK reference in commit message
>
>Suggested-by: Niranjana Vishwanathapura <niranjana.vishwanathapura at intel.com>
>Signed-off-by: Matthew Brost <matthew.brost at intel.com>

This patch should probably go in the 'USM locking fixes' patch series?

>---
> drivers/gpu/drm/xe/xe_vm.c | 15 +++++++--------
> 1 file changed, 7 insertions(+), 8 deletions(-)
>
>diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
>index 68ab51f84bed..ab036a51d17e 100644
>--- a/drivers/gpu/drm/xe/xe_vm.c
>+++ b/drivers/gpu/drm/xe/xe_vm.c
>@@ -1349,6 +1349,13 @@ void xe_vm_close_and_put(struct xe_vm *vm)
> 	XE_WARN_ON(!list_empty(&vm->extobj.list));
> 	up_write(&vm->lock);
>
>+	mutex_lock(&xe->usm.lock);
>+	if (vm->flags & XE_VM_FLAG_FAULT_MODE)
>+		xe->usm.num_vm_in_fault_mode--;
>+	else if (!(vm->flags & XE_VM_FLAG_MIGRATION))
>+		xe->usm.num_vm_in_non_fault_mode--;
>+	mutex_unlock(&xe->usm.lock);
>+
> 	xe_vm_put(vm);

My concern is that something might be holding VM reference and hence
VM will continue to live after we decrement the counts here and thus
might endup in a new VM created in fault mode while the old VM in
non-fault mode still not freed (and vice versa). Is that fine?

Niranjana

> }
>
>@@ -1393,18 +1400,10 @@ static void vm_destroy_work_func(struct work_struct *w)
>
> 	drm_gpuva_manager_destroy(&vm->mgr);
>
>-	mutex_lock(&xe->usm.lock);
>-	if (vm->flags & XE_VM_FLAG_FAULT_MODE)
>-		xe->usm.num_vm_in_fault_mode--;
>-	else if (!(vm->flags & XE_VM_FLAG_MIGRATION))
>-		xe->usm.num_vm_in_non_fault_mode--;
>-	mutex_unlock(&xe->usm.lock);
>-
> 	trace_xe_vm_free(vm);
> 	dma_fence_put(vm->rebind_fence);
> 	dma_resv_fini(&vm->resv);
> 	kfree(vm);
>-
> }
>
> void xe_vm_free(struct kref *ref)
>-- 
>2.34.1
>