Date: Wed, 10 Nov 2004 16:33:00 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Karsten Wiese <annabellesgarden@yahoo.de>
Cc: linux-kernel@vger.kernel.org, Lee Revell <rlrevell@joe-job.com>,
       Rui Nuno Capela <rncbc@rncbc.org>, Mark_H_Johnson@Raytheon.com,
       "K.R. Foley" <kr@cybsft.com>, Bill Huey <bhuey@lnxw.com>,
       Adam Heath <doogie@debian.org>, Florian Schmidt <mista.tapas@gmx.net>,
       Thomas Gleixner <tglx@linutronix.de>,
       Michal Schmidt <xschmi00@stud.feec.vutbr.cz>,
       Fernando Pablo Lopez-Lezcano <nando@ccrma.Stanford.EDU>,
       Gunther Persoons <gunther_persoons@spymac.com>, emann@mrv.com,
       Shane Shrybman <shrybman@aei.ca>, Amit Shah <amit.shah@codito.com>
Subject: Re: [patch] Real-Time Preemption, -RT-2.6.10-rc1-mm3-V0.7.23
Message-ID: <20041110153300.GB9875@elte.hu>
References: <20041021132717.GA29153@elte.hu> <200411101452.36007.annabellesgarden@yahoo.de> <20041110150136.GA8668@elte.hu> <200411101520.43192.annabellesgarden@yahoo.de>
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* Karsten Wiese <annabellesgarden@yahoo.de> wrote:

> Yes, it always happens, when callling ./cvscompile script of a
> project, that is mounted via nfs. Haven't tried to do that
> ./cvscompile locally, should I?

very interesting, nfs is indeed one of the frequent BKL users.

a 'BKL leak' is an unbalanced lock, e.g.:

	lock_kernel();
	... do stuff ...
	if (error)
		return;
	... do more stuff ...
	unlock_kernel();

the BKL is a very special type of lock which fact has the side-effect
that in the stock kernel a 'BKL leak' can go unnoticed very easily: it
causes no problems other than hard-to-debug (but severe) scalability
regressions. The moment the BKL count leaked from the NFS code that
process has been 'scalability-poisoned' and will be handicapped until it
exits.

In the PREEMPT_RT patchset i added a strict locking checker to do_exit()
that found this apparent NFS bug. Unfortunately the deadlock detector
only reported a pretty common place where the BKL gets
dropped/reacquired frequently so we dont know where the NFS code has the
lock/unlock imbalance. Could you report this bug to the NFS maintainers
(along with the above and the previous analysis)?

	Ingo
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