linux-kernel - Re: [PATCH tip/core/rcu 09/12] rcu: Make call_rcu_tasks() tolerate first call with irqs disabled

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Message-Id: <20160615225813.GC3923@linux.vnet.ibm.com>
Date:	Wed, 15 Jun 2016 15:58:13 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	jiangshanlai@...il.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH tip/core/rcu 09/12] rcu: Make call_rcu_tasks() tolerate
 first call with irqs disabled

On Thu, Jun 16, 2016 at 12:16:04AM +0200, Peter Zijlstra wrote:
> On Wed, Jun 15, 2016 at 02:46:10PM -0700, Paul E. McKenney wrote:
> > Currently, if the very first call to call_rcu_tasks() has irqs disabled,
> > it will create the rcu_tasks_kthread with irqs disabled, which will
> > result in a splat in the memory allocator, which kthread_run() invokes
> > with the expectation that irqs are enabled.
> > 
> > This commit fixes this problem by deferring kthread creation if called
> > with irqs disabled.  The first call to call_rcu_tasks() that has irqs
> > enabled will create the kthread.
> > 
> > This bug was detected by rcutorture changes that were motivated by
> > Iftekhar Ahmed's mutation-testing efforts.
> 
> Seems fragile. What if someone manages to only use call_rcu_tasks() with
> IRQs disabled?

It would have to have users before that could possibly happen.  :-/
And it would not be hard to remove the fragility if needed by setting
up a workqueue, possibly mediated by a timer or whatever.  But it is
hard to motivate myself to do so in advance of users.  For that matter...

Steven, is call_rcu_tasks() needed, or should I just rip it out?

							Thanx, Paul