linux-kernel - Re: [PATCH rcu/urgent] Banishing kthreads

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Date:	Fri, 17 Jun 2011 14:00:39 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	patches@...aro.org
Subject: Re: [PATCH rcu/urgent] Banishing kthreads

On Fri, Jun 17, 2011 at 04:35:23PM -0400, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@...ux.vnet.ibm.com) wrote:
> > This patchset banishes RCU kthreads from non-RCU_BOOST kernel threads.
> > The two patches are as follows:
> > 
> > 1.	Minimal patch that #ifdefs out the kthread code.
> > 
> > 2.	Code-movement patch that puts the code #ifdefed out above
> > 	under existing #ifdefs in kernel/rcutree_plugin.h.
> 
> Hi Paul,
> 
> I'm wondering about the impact of this change: so I guess that before
> the change, it was OK to go on a waitqueue (might_sleep()) within a
> call_rcu callback, but since the execution now moves to a softirq
> handler in non-RCU_BOOST kernels, it's not allowed anymore. I might be
> missing something though: was sleeping within call_rcu handlers already
> prohibited ? (never had to sleep in those, so I never had to check if it
> was allowed)

The callbacks are still executed in BH context, so any attempt to
sleep should get a warning message.  Also, it switched to kthread quite
recently, so there hasn't been time for anyone to get used to it.  ;-)

							Thanx, Paul
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