linux-kernel - Re: [PATCH] sched: Use RCU read lock on all calls to dl_bw

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Date:	Mon, 29 Sep 2014 18:54:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kirill Tkhai <ktkhai@...allels.com>
Cc:	Sasha Levin <sasha.levin@...cle.com>, mingo@...nel.org,
	torvalds@...ux-foundation.org, paulmck@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: Use RCU read lock on all calls to dl_bw_of()

On Mon, Sep 29, 2014 at 08:43:47PM +0400, Kirill Tkhai wrote:
> Thanks for your report. It looks like your fix is not enough, because
> we check for rcu_read_lock_sched_held() in dl_bw_of(). It still warns
> even if rcu_read_lock() is held.
> 
> I used rcu_read_lock_sched_held() because we free root_domain using
> call_rcu_sched(). So, it's necessary to held rcu_read_lock_sched(),
> and my initial commit has this problem too.
> 
> It looks like we should fix it in a way like this:
> 
> [PATCH]sched: Use dl_bw_of() under rcu_read_lock_sched()
> 
> rq->rd is freed using call_rcu_sched(), and it's accessed with preemption
> disabled in the most cases.
> 
> So in other places we should use rcu_read_lock_sched() to access it to fit
> the scheme:
> 
> rcu_read_lock_sched() or preempt_disable() <==> call_rcu_sched().

Hmm, sad that. I cannot remember why that is rcu_sched, I suspect
because we rely on it someplace but I cannot remember where.

We could of course do a double take on that and use call_rcu after
call_rcu_sched(), such that either or both are sufficient.

I would very much prefer not to add extra preempt_disable()s if
possible.
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