linux-kernel - Re: [PATCH 3/3] wait_task_inactive: don't use the dummy version when !SMP && PREEMPT

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Message-ID: <b647ffbd0807301246s1def837ar8c389a3d3f7ec2cc@mail.gmail.com>
Date:	Wed, 30 Jul 2008 21:46:48 +0200
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	"Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:	"Oleg Nesterov" <oleg@...sign.ru>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Roland McGrath" <roland@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] wait_task_inactive: don't use the dummy version when !SMP && PREEMPT

2008/7/30 Linus Torvalds <torvalds@...ux-foundation.org>:
>
>
> On Wed, 30 Jul 2008, Oleg Nesterov wrote:
>>
>> The patch looks monstrous because it moves the (unchanged) definition
>> of wait_task_inactive() outside of "#ifdef CONFIG_SMP", but it is quite
>> trivial.
>
> Hmm. Doesn't this just deadlock in UP (PREEMPT) if wait_task_interactive()
> is ever called from a no-preempt context?
>
> And if that's never the case, the comment should be updated to reflect
> that (right now it says that it's only invalid to call it with interrupts
> disabled to avoid cross-IPI deadlocks).
>
> Oh, and shouldn't it do a "yield()" instead of a cpu_relax() on UP?

This part could have been skipped for UP. task_running(rq, p) just
can't give 'true' for UP (otherwise it's a bug). The only relevant
part is "on_rq = p->se.on_rq".

>
> Inquiring minds want to know. That function was very much expressly
> designed for SMP,

It looks so. Otherwise it's behavior is not symmetric and I think,
either [1] it shouldn't be a "nop" for !SMP or [2] there shouldn't be
a version for !SMP at all --> so no one can make false assumptions.

(if [1], then I think a separate function for PREEMPT would look
better, i.e. without parts with task_running())

e.g. consider this code from kthread_bind():

        /* Must have done schedule() in kthread() before we set_task_cpu */
        wait_task_inactive(k, 0);

        set_task_cpu(k, cpu);
        k->cpus_allowed = cpumask_of_cpu(cpu);
        k->rt.nr_cpus_allowed = 1;
        k->flags |= PF_THREAD_BOUND;

set_task_cpu(k, cpu) is safe _only_ if 'k' is not on the run-queue
(and can't be placed onto it behind our back -- heh, a bit subtle).

Now, for !SMP + PREEMPT it's not a case. set_task_cpu() may be called
while 'k' is still on the run-queue (more precisely, preempted in
kthread() between complete(&create->started); and schedule();).

Yes, set_task_cpu() is a "nop" for UP so that's ok in this particular
case. But let's suppose, another use-case would be introduced with
'false' assumptions causing troubles for !SMP.

> not for preemption, and I want to understand why it's
> ok (_if_ it's ok).
>
>                Linus

-- 
Best regards,
Dmitry Adamushko
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