linux-kernel - Re: CPUFREQ-CPUHOTPLUG: Possible circular locking dependency

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Message-ID: <20061201014313.GA25074@in.ibm.com>
Date:	Fri, 1 Dec 2006 07:13:13 +0530
From:	Gautham R Shenoy <ego@...ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Gautham R Shenoy <ego@...ibm.com>, Andrew Morton <akpm@...l.org>,
	linux-kernel@...r.kernel.org, torvalds@...l.org, davej@...hat.com,
	dipankar@...ibm.com, vatsa@...ibm.com
Subject: Re: CPUFREQ-CPUHOTPLUG: Possible circular locking dependency

On Thu, Nov 30, 2006 at 09:29:34AM +0100, Ingo Molnar wrote:
> what lockdep does is it observes actual locking dependencies as they
> happen individually in various contexts, and then 'completes' the
> dependency graph by combining all the possible scenarios how contexts
> might preempt each other. So if lockdep sees independent dependencies
> and concludes that they are circular, there's nothing that saves us from
> the deadlock.

Ingo,

Consider a case where we have three locks A, B and C.
We have very clear locking rule inside the kernel that lock A *should*
be acquired before acquiring either lock B or lock C.

At runtime lockdep detects the two dependency chains,
A --> B --> C

and

A --> C --> B.

Does lockdep issue a circular dependency warning for this ? 
It's quite clear from the locking rule that we cannot have a
circular deadlock, since A acts as a mutex for B->C / C->B callpath.

Just curious :-) [ Well, I might encounter such a scenario in an attempt
		   to make cpufreq cpu-hotplug safe! ]

> 	Ingo

Thanks and Regards
gautham.
-- 
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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