linux-kernel - Re: [tip:timers/core] clocksource: Resolve cpu hotplug dead lock with TSC unstable

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Message-ID: <20090831143849.GA10603@elte.hu>
Date:	Mon, 31 Aug 2009 16:38:49 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	johnstul@...ibm.com, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:timers/core] clocksource: Resolve cpu hotplug dead lock
	with TSC unstable


* Martin Schwidefsky <schwidefsky@...ibm.com> wrote:

> On Fri, 28 Aug 2009 18:34:00 GMT
> tip-bot for Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> > Commit-ID:  7285dd7fd375763bfb8ab1ac9cf3f1206f503c16
> > Gitweb:     http://git.kernel.org/tip/7285dd7fd375763bfb8ab1ac9cf3f1206f503c16
> > Author:     Thomas Gleixner <tglx@...utronix.de>
> > AuthorDate: Fri, 28 Aug 2009 20:25:24 +0200
> > Committer:  Thomas Gleixner <tglx@...utronix.de>
> > CommitDate: Fri, 28 Aug 2009 20:25:24 +0200
> > 
> > clocksource: Resolve cpu hotplug dead lock with TSC unstable
> > 
> > Martin Schwidefsky analyzed it:
> > To register a clocksource the clocksource_mutex is acquired and if
> > necessary timekeeping_notify is called to install the clocksource as
> > the timekeeper clock. timekeeping_notify uses stop_machine which needs
> > to take cpu_add_remove_lock mutex.
> > Starting a new cpu is done with the cpu_add_remove_lock mutex held.
> > native_cpu_up checks the tsc of the new cpu and if the tsc is no good
> > clocksource_change_rating is called. Which needs the clocksource_mutex
> > and the deadlock is complete.
> > 
> > The solution is to replace the TSC via the clocksource watchdog
> > mechanism. Mark the TSC as unstable and schedule the watchdog work so
> > it gets removed in the watchdog thread context.
> > 
> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> > LKML-Reference: <new-submission>
> > Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
> > Cc: John Stultz <johnstul@...ibm.com>
> 
> Ah, very good. I've been going round in circles to find a solution 
> that allows to downgrade the tsc rating when the second cpu is 
> enabled. Could not find a solution. Your approach changes 
> semantics slightly: the tsc clock will continue with its old 
> rating for a while until the watchdog will do the downgrade. If 
> that is acceptable then this is a good solution.

Latest timers/core also passed thousands of iterations of -tip 
testing so far, so that painful series of locking and stability 
troubles has been solved and the bits look good for v2.6.32.

	Ingo
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