linux-kernel - Re: CFS rq lock question

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Message-Id: <1207935421.7524.3.camel@twins>
Date:	Fri, 11 Apr 2008 19:37:01 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Dan Upton <upton.dan.linux@...il.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: CFS rq lock question

On Fri, 2008-04-11 at 13:21 -0400, Dan Upton wrote:
> I'm poking around with some scheduler stuff, and there's something I'm
> not clear on for the CFS runqueue locks.  The comments before
> __load_balance_iterator(...) in sched_fair.c suggests things can be
> dequeued even though the runqueue lock is held.  Can things also be
> added to the queue while the lock is held?  (Also, either way, what's
> the rationale that dequeueing is a safe procedure when somebody else
> holds a lock?)

/*
 * Load-balancing iterator. Note: while the runqueue stays locked
 * during the whole iteration, the current task might be
 * dequeued so the iterator has to be dequeue-safe. Here we
 * achieve that by always pre-iterating before returning
 * the current task:
 */

I don't think this comment is correct, but if it were, it would only
apply to rq->curr, not for any enqueue/dequeue.



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