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

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Date:	Mon, 15 Mar 2010 10:09:58 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>
Cc:	Ben Blum <bblum@...gle.com>, Jiri Slaby <jirislaby@...il.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Miao Xie <miaox@...fujitsu.com>,
	Paul Menage <menage@...gle.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 0/6] sched/cpusets fixes, more changes are needed

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.

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