linux-kernel - Re: [PATCH RT] sched/migrate_disable: fallback to preempt

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Date:   Thu, 5 Jul 2018 18:59:37 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Joe Korty <joe.korty@...current-rt.com>
Cc:     Julia Cartwright <julia@...com>, tglx@...utronix.de,
        rostedt@...dmis.org, linux-rt-users@...r.kernel.org,
        linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH RT] sched/migrate_disable: fallback to preempt_disable()
 instead barrier()

On 2018-07-05 12:18:07 [-0400], joe.korty@...current-rt.com wrote:
> Hi Sebastian,
Hi Joe,

> I just verified that this fix does not work for my mix of
> config options (smp && preempt && !rt).

Okay. So for !RT+SMP we keep migrate_disable() around and it almost
nothing. And it is not referenced anywhere so it does not matter as long
as it not used directly.

We could turn migrate_disable() into a nop/barrier but then we have
three uses which do preempt_disable() -> migrate_disable() (see other
thread).
For the futex code it should not matter much because at this point
preemption is disabled due to the spin_lock() (so we would just extend
it past the spin_unlock() or wake_futex_pi() which ends with
preempt_enable()).

> Regards,
> Joe

Sebastian