Re: keventd_create_kthread

[Rusty Russell]

In message <Pine.LNX.4.58.0402190205040.16515@xxxxxxxxxxxxxxxxxxxxxxxx> you write:
> 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.

No, because it can come back at any time 8(

> that in any modern interface. Why does keventd_create_kthread() need
> wait_task_inactive()?

Um, the code was taken from sched.c to kthread:

2.6.3 migration_thread():
	set_current_state(TASK_UNINTERRUPTIBLE);
	schedule();

2.6.3 migration_call():
		kernel_thread(migration_thread, &startup, CLONE_KERNEL);
		wait_for_completion(&startup.startup_done);
		wait_task_inactive(startup.task);

		startup.task->thread_info->cpu = cpu;
		startup.task->cpus_allowed = cpumask_of_cpu(cpu);


So, if the migration thread has been preempted immediately before
schedule(), wait_task_inactive returns, but it can come back from
preempt while we're messing with startup.task->thread_info->cpu.

Now, the latter part is wrapped in kthread_bind(), which should really
be doing the wait_task_inactive itself (doing it in kthread_create is
overzealous).  But the race is still there.

Hope that clarifies,
Rusty.
--
  Anyone who quotes me in their sig is an idiot. -- Rusty Russell.
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