Re: [PATCH RT] Defer migrate_enable migration while task state != TASK_RUNNING

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 23 Mar 2018 13:21:31 -0400
joe.korty@concurrent-rt.com wrote:

> My understanding is, in standard Linux and in rt, setting
> task state to anything other than TASK_RUNNING in of itself
> blocks preemption.

That is clearly false. The only thing that blocks preemption with a
CONFIG_PREEMPT kernel is preempt_disable() and local_irq*() disabling.

(Note spin_locks call preempt_disable in non RT).

Otherwise, nothing will stop preemption.

>  A preemption is not really needed here
> as it is expected that there is a schedule() written in that
> will shortly be executed.  And if a 'involuntary schedule'
> (ie, preemption) were allowed to occur between the task
> state set and the schedule(), that would change the task
> state back to TASK_RUNNING, which would cause the schedule
> to NOP.  Thus we risk not having paused long enough here
> for the condition we were waiting for to become true.

That is also incorrect. As Julia mentioned, a preemption keeps the
state of the task.

-- Steve