linux-kernel - Re: [PATCH 5/8] rcu: eliminate deadlock for rcu read site

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Message-ID: <20130812135544.GG27162@twins.programming.kicks-ass.net>
Date:	Mon, 12 Aug 2013 15:55:44 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	paulmck@...ux.vnet.ibm.com, Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, Dipankar Sarma <dipankar@...ibm.com>
Subject: Re: [PATCH 5/8] rcu: eliminate deadlock for rcu read site

On Fri, Aug 09, 2013 at 05:31:27PM +0800, Lai Jiangshan wrote:
> On 08/09/2013 04:40 AM, Paul E. McKenney wrote:
> > One problem here -- it may take quite some time for a set_need_resched()
> > to take effect.  This is especially a problem for RCU priority boosting,
> > but can also needlessly delay preemptible-RCU grace periods because
> > local_irq_restore() and friends don't check the TIF_NEED_RESCHED bit.
> 
> 
> The final effect of deboosting(rt_mutex_unlock()) is also accomplished
> via set_need_resched()/set_tsk_need_resched().
> set_need_resched() is enough for RCU priority boosting issue here.

But there's a huge difference between the boosting and deboosting side
of things. rcu_read_unlock_special() starts the boost, the deboosting
only matters if/when you reschedule. 
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