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

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Message-ID: <20090831101928.4c00c797@skybase>
Date:	Mon, 31 Aug 2009 10:19:28 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	johnstul@...ibm.com, schwidefsky@...ibm.com, tglx@...utronix.de
Cc:	tglx@...utronix.de, linux-tip-commits@...r.kernel.org,
	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...hat.com,
	johnstul@...ibm.com
Subject: Re: [tip:timers/core] clocksource: Resolve cpu hotplug dead lock
 with TSC unstable

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.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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