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: <20160615221604.GC30921@twins.programming.kicks-ass.net>
Date:	Thu, 16 Jun 2016 00:16:04 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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 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?