linux-kernel - Re: [PATCH RFC tip/core/rcu 2/2] rcu: Create rcuo kthreads only for onlined CPUs

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Message-ID: <53C91CA9.7060103@oracle.com>
Date:	Fri, 18 Jul 2014 09:10:01 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
	oleg@...hat.com, sbw@....edu, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH RFC tip/core/rcu 2/2] rcu: Create rcuo kthreads only for
 onlined CPUs

On 07/18/2014 08:55 AM, Paul E. McKenney wrote:
> On Fri, Jul 18, 2014 at 07:17:17AM -0400, Sasha Levin wrote:
>> > On 07/14/2014 06:06 AM, Paul E. McKenney wrote:
>>> > > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
>>> > > 
>>> > > RCU currently uses for_each_possible_cpu() to spawn rcuo kthreads,
>>> > > which can result in more rcuo kthreads than one would expect, for
>>> > > example, derRichard reported 64 CPUs worth of rcuo kthreads on an
>>> > > 8-CPU image.  This commit therefore creates rcuo kthreads only for
>>> > > those CPUs that actually come online.
>>> > > 
>>> > > Reported-by: derRichard
>>> > > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>> > 
>> > Hey Paul,
>> > 
>> > Me again. :)
>> > 
>> > It seems that this patch moved thread initialization to a point way
>> > too early during boot, before rest_init() which initializes kthreadd_task
>> > runs, so creating a new kthread triggers a NULL ptr deref:
> This should be fixed by commit 918179699e4a in -rcu.  My guess is that
> you are instead using commit c6e2955266d14, which, said to say, made it
> into -next yesterday.  :-(
> 
> If my guess is wrong, please let me know!

Your guess is correct.

I'll wait for the next -next tree and retry.


Thanks,
Sasha
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