linux-kernel - Re: [GIT PULL] RCU fix

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Message-ID: <BANLkTimapK+1vDwB-Akw0eVimM4gHBdqaw@mail.gmail.com>
Date:	Wed, 1 Jun 2011 02:52:59 +0900
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] RCU fix

On Wed, Jun 1, 2011 at 2:44 AM, Paul E. McKenney
<paulmck@...ux.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
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