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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 1 Jun 2011 02:52:59 +0900
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Subject: Re: [GIT PULL] RCU fix
To: paulmck@linux.vnet.ibm.com
Cc: Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>
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On Wed, Jun 1, 2011 at 2:44 AM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
>
> The reason for the switch is to allow threads blocked in TREE_PREEMPT_RCU
> and TINY_PREEMPT_RCU RCU read-side critical sections to have their
> priority boosted in order to avoid OOM.  People have made these OOMs
> happen, so this is not longer just a theoretical concern.

Quite frankly, that doesn't make much sense.

First off, the default for priority boosting is off (and you cannot
even select it unless you have RT_MUTEX and PREEMPT_RCU), so why the
heck do we still use the threads even when we don't support the
boosting at all?

Secondly, if a process is in danger of exhausting the RCU resources,
and it is preemptable, why doesn't the rcu_read_unlock() logic just
try to force a reschedule and thus an rcu idle period? Using processes
and process priorities for this seems to be just stupid.

I dunno. After RCU_TINY showed how fragile it was to use kernel
threads for this, and after this subtle issue just re-inforced that
conclusion, I just cannot begin to believe that using a thread was the
right thing to do. It just seems stupid.

                         Linus