Re: keventd_create_kthread

[Ingo Molnar]

On Thu, 19 Feb 2004, Rusty Russell wrote:

> That's not enough: it can set that and then get preemted.  It really
> want to return when the task is off the runqueue.  The original
> wait_task_inactive() does an incredible complicated and AFAICT useless
> dance wrt not locking and disabling preempt explicitly.  Ingo, how's
> this replacement?  (And who wrote this code?)

this is old code that morphed many times. Its main use was for exit.c's
purpose and in heavy clone/exit workloads it made quite a difference
whether the 'polling' for task exit was done under the runqueue lock or
not - hence the complexity. Task freeing is poll-free in 2.6 so
wait_task_inactive() doesnt get nearly as heavy use.

The current wait_task_inactive() code seems to be OK on x86.
Context-switching cannot be preempted. The goal of wait_task_inactive() is
to wait for the task to unschedule on a CPU. If that's due to preempt then
it's due to preempt.

i'd strongly advise against using wait_task_inactive() in
keventd_create_kthread() - it's _polling_. We must not do any polling like
that in any modern interface. Why does keventd_create_kthread() need
wait_task_inactive()?

	Ingo
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