linux-kernel - Re: Possible lock-less list race in scheduler

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Date:	Fri, 6 Mar 2015 15:39:25 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Huang Ying <ying.huang@...el.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Lai Jiangshan <eag0628@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: Possible lock-less list race in scheduler_ipi()

On Fri, 6 Mar 2015 15:38:21 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Fri, 6 Mar 2015 19:39:44 +0000 (UTC)
> Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> 
> ask concurrently with the llist iteration within sched_ttwu_pending().
> > 
> > AFAIU, ttwu_queue_remote() is called from ttwu_queue() without holding
> > the rq lock. So I'm wondering what prevents corruption of the wake_list
> > in this situation.
> 
> I guess if it is on the wake_list, then the task's state is already
> RUNNING. Any other task can switch a task's state to RUNNING but only
> the task itself can switch it back to something else. If the task is on
> the wake_list, it's state is already RUNNING, but it has not run yet.
> That means any other wakeup will jump to the "goto out" and skip over
> the ttwu_queue() call.

If my assumption is indeed the case, then these types of subtleties
really need comments in the code.

-- Steve
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