Subject: Re: [PATCH net-next 2/2] selftests: net: py: ensure defer() is only used within a test case

From: Petr Machata <[email-masked]>
Date: Fri, 9 Jan 2026 09:23:54 +0100

Jakub Kicinski <[email-masked]> writes:

> I wasted a couple of hours recently after accidentally adding
> a defer() from within a function which itself was called as
> part of defer(). This leads to an infinite loop of defer().
> Make sure this cannot happen and raise a helpful exception.
>
> I understand that the pair of _ksft_defer_arm() calls may
> not be the most Pythonic way to implement this, but it's
> easy enough to understand.
>
> Signed-off-by: Jakub Kicinski <[email-masked]>

I think we achieve the same without the extra globals though? Just drain
the queue and walk through a copy of it?

    defer_queue = utils.GLOBAL_DEFER_QUEUE
    utils.GLOBAL_DEFER_QUEUE = []
    for i, entry in enumerate(defer_queue):
        ...
    if utils.GLOBAL_DEFER_QUEUE:
        warning / exception

> ---
>  tools/testing/selftests/net/lib/py/ksft.py  | 7 +++++++
>  tools/testing/selftests/net/lib/py/utils.py | 3 +++
>  2 files changed, 10 insertions(+)
>
> diff --git a/tools/testing/selftests/net/lib/py/ksft.py b/tools/testing/selftests/net/lib/py/ksft.py
> index 248cd1a723a3..0a96f88bb60a 100644
> --- a/tools/testing/selftests/net/lib/py/ksft.py
> +++ b/tools/testing/selftests/net/lib/py/ksft.py
> @@ -153,6 +153,11 @@ KSFT_DISRUPTIVE = True
>      print(res, flush=True)
>  
>  
> +def _ksft_defer_arm(state):
> +    """ Allow or disallow the use of defer() """
> +    utils.GLOBAL_DEFER_ARMED = state
> +
> +
>  def ksft_flush_defer():
>      global KSFT_RESULT
>  
> @@ -315,6 +320,7 @@ KsftCaseFunction = namedtuple("KsftCaseFunction",
>          comment = ""
>          cnt_key = ""
>  
> +        _ksft_defer_arm(True)
>          try:
>              func(*args)
>          except KsftSkipEx as e:
> @@ -332,6 +338,7 @@ KsftCaseFunction = namedtuple("KsftCaseFunction",
>                  ksft_pr(f"Stopping tests due to {type(e).__name__}.")
>              KSFT_RESULT = False
>              cnt_key = 'fail'
> +        _ksft_defer_arm(False)
>  
>          try:
>              ksft_flush_defer()
> diff --git a/tools/testing/selftests/net/lib/py/utils.py b/tools/testing/selftests/net/lib/py/utils.py
> index 2dde34560d65..824f039d384c 100644
> --- a/tools/testing/selftests/net/lib/py/utils.py
> +++ b/tools/testing/selftests/net/lib/py/utils.py
> @@ -142,6 +142,7 @@ import time
>  
>  
>  GLOBAL_DEFER_QUEUE = []
> +GLOBAL_DEFER_ARMED = False
>  
>  
>  class defer:
> @@ -153,6 +154,8 @@ GLOBAL_DEFER_QUEUE = []
>          self.args = args
>          self.kwargs = kwargs
>  
> +        if not GLOBAL_DEFER_ARMED:
> +            raise Exception("defer queue not armed, did you use defer() outside of a test case?")
>          self._queue = GLOBAL_DEFER_QUEUE
>          self._queue.append(self)