[PATCH 0/6] sched/cpusets fixes, more changes are needed

[Oleg Nesterov <oleg@redhat.com>]

Ingo, Peter.

Unless I missed something, with or without these patches the TASK_WAKING
logic in do_fork() is very broken.

	- do_fork() clears PF_STARTING and then calls wake_up_new_task()
	  which finally does s/WAKING/RUNNING.

	  But. Nobody can take rq->lock in between. This means a signal
	  from irq (quite possible with CLONE_THREAD) or another rt
	  thread which preempts us can lockup.

	- the comment in wake_up_new_task says:
	
		We still have TASK_WAKING but PF_STARTING is gone now, meaning
		->cpus_allowed is stable

	  this is not true. Yes, nobody can take rq->lock _after_ we cleared
	  PF_STARTING, but it is possible that another thread took this lock
	  before and still holds it doing, say, sched_setaffinity().

No?

If yes. I can make a patch, but the question is: what is the point to use
TASK_WAKING in fork pathes? Can't sched_fork() set TASK_RUNNING instead?
Afaics, TASK_RUNNING can equally protect from premature wakeups but doesn't
these PF_STARTING complications.

As for this series. Please review. I don't understand how it is possible
to really test these changes.

Dear cpuset developers! Please review ;) If you don't like 6/6, please make
a better fix. I tried to make as "simple" patch as possible because I hardly
understand cpuset.c, last time I quickly read it a long ago.

Oleg.