linux-kernel - Re: [RFC PATCH V2] rt/aio: fix rcu garbage collection might

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Message-ID: <1402459804.5195.19.camel@marge.simpson.net>
Date:	Wed, 11 Jun 2014 06:10:04 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Benjamin LaHaise <bcrl@...ck.org>
Cc:	Kent Overstreet <kmo@...erainc.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	RT <linux-rt-users@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH V2] rt/aio: fix rcu garbage collection might_sleep()
 splat

On Tue, 2014-06-10 at 13:50 -0400, Benjamin LaHaise wrote: 
> On Tue, Jun 10, 2014 at 05:47:28AM +0200, Mike Galbraith wrote:
> > On Mon, 2014-06-09 at 10:08 +0800, Lai Jiangshan wrote: 
> > > Hi, rt-people
> > > 
> > > I don't think it is the correct direction.
> > > Softirq (including local_bh_disable()) in RT kernel should be preemptible.
> > 
> > How about the below then?
> > 
> > I was sorely tempted to post a tiny variant that dropped taking ctx_lock
> > in free_ioctx_users() entirely, as someone diddling with no reference
> > didn't make sense.  Cc Ben, he would know.
> 
> That should be okay...  Let's ask Kent to chime in on whether this looks 
> safe to him on the percpu ref front as well, since he's the one who wrote 
> this code.

Looking at the gizzard of our in-tree user of kiocb_set_cancel_fn()
(gadget), cancel() leads to dequeue() methods, which take other sleeping
locks, so tiniest variant is not an option, patchlet stands.

-Mike

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