Re: [PATCH v3] workqueue: Fix double kfree for rescuer

Re: [PATCH v3] workqueue: Fix double kfree for rescuer

[Markus Elfring]

> The callback function "rcu_free_wq" could be called after memory
> was released for "rescuer" already, Thus delete a misplaced call
> of the function "kfree".

I got into the mood to follow your interpretation of the
software situation for a moment.

I have taken another look also at the implementation of the function “destroy_workqueue”.

* The function call “kfree(rescuer)” can be performed there in an if branch
  after the statement “wq->rescuer = NULL” was executed.

* This data processing is independent from a possible call of the
  function “call_rcu(&wq->rcu, rcu_free_wq)” in another if branch.
  Thus it seems that a null pointer is intentionally passed by a data structure
  member to this callback function on demand.
  The corresponding call of the function “kfree” can tolerate this special case.


Now I find that the proposed change can be inappropriate.
I'm sorry for undesirable confusion because of this patch review.

Regards,
Markus

Re: [PATCH v3] workqueue: Fix double kfree for rescuer

[Markus Elfring]

> The callback function "rcu_free_wq" could be called after memory
> was released for "rescuer" already, Thus delete a misplaced call
> of the function "kfree".

I got into the mood to follow your interpretation of the
software situation for a moment.

I have taken another look also at the implementation of the function “destroy_workqueue”.

* The function call “kfree(rescuer)” can be performed there in an if branch
  after the statement “wq->rescuer = NULL” was executed.

* This data processing is independent from a possible call of the
  function “call_rcu(&wq->rcu, rcu_free_wq)” in another if branch.
  Thus it seems that a null pointer is intentionally passed by a data structure
  member to this callback function on demand.
  The corresponding call of the function “kfree” can tolerate this special case.


Now I find that the proposed change can be inappropriate.
I'm sorry for undesirable confusion because of this patch review.

Regards,
Markus

Re: [v3] workqueue: Fix double kfree for rescuer

[Markus Elfring]

> wq->rescuer is guaranteed to be NULL in rcu_free_wq()

I was unsure about this data processing detail.


> The patch is a cleanup to remove a "kfree(NULL);".

I would prefer also an improved commit message according to
the understanding of the software situation in this direction.

Regards,
Markus

Re: [v3] workqueue: Fix double kfree for rescuer

[Markus Elfring]

> There is something wrong with my description.  is it feasible to describe as follows:
>
> The resucer is already free in "destroy_workqueue" and
>     "wq->rescuer = NULL" was executed, but in "rcu_free_wq"
>     it's release again (equivalent to kfree(NULL)), this is
>     unnecessary, so should remove.

I find that this suggestion indicates further wording challenges.
Please try another variant.

Regards,
Markus