linux-kernel - Re: 2.6.26-rc9-git4: Reported regressions from 2.6.25

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Message-Id: <200807111412.00084.nickpiggin@yahoo.com.au>
Date:	Fri, 11 Jul 2008 14:11:59 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Adrian Bunk <bunk@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Natalie Protasevich <protasnb@...il.com>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ibm.com>
Subject: Re: 2.6.26-rc9-git4: Reported regressions from 2.6.25

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...
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