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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Ingo Molnar <mingo@elte.hu>
Subject: Re: 2.6.26-rc9-git4: Reported regressions from 2.6.25
Date: Fri, 11 Jul 2008 14:11:59 +1000
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Cc: Linus Torvalds <torvalds@linux-foundation.org>,
       Alexey Dobriyan <adobriyan@gmail.com>,
       "Rafael J. Wysocki" <rjw@sisk.pl>,
       Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Adrian Bunk <bunk@kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Natalie Protasevich <protasnb@gmail.com>,
       Kernel Testers List <kernel-testers@vger.kernel.org>,
       "Paul E. McKenney" <paulmck@us.ibm.com>
References: <4yrA-dRoNlD.A.jcB.ny-cIB@albercik> <20080710173459.GA11648@elte.hu> <20080710180620.GA30844@elte.hu>
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On Friday 11 July 2008 04:06, Ingo Molnar wrote:

> i'm wondering why rcutorture didnt trigger it. I do run !HOTPLUG +
> RCU_PREEMPT kernels and never saw this. Nor did Paul. That aspect is
> weird.

It basically requires an active rcu reader to be preempted (preferably
by something doing a lot of call_rcu or other activity ie. the writer
so it can tick along the different states quickly).

I found just 2 threads (reader and writer) bound to the same CPU would
trigger it fastest, my reader has quite a long rcu read section.

I'm not sure why rcutorture doesn't trigger for everyone. I'm surprised
it does not have much longer maximum read delays -- several ms I would
have thought should be useful to have a crticial section open while the
rcu engine can run through a number of states...