Date: Fri, 1 Dec 2006 07:13:13 +0530
From: Gautham R Shenoy <ego@in.ibm.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: Gautham R Shenoy <ego@in.ibm.com>, Andrew Morton <akpm@osdl.org>,
       linux-kernel@vger.kernel.org, torvalds@osdl.org, davej@redhat.com,
       dipankar@in.ibm.com, vatsa@in.ibm.com
Subject: Re: CPUFREQ-CPUHOTPLUG: Possible circular locking dependency
Message-ID: <20061201014313.GA25074@in.ibm.com>
Reply-To: ego@in.ibm.com
References: <20061129152404.GA7082@in.ibm.com> <20061129130556.d20c726e.akpm@osdl.org> <20061130042807.GA4855@in.ibm.com> <20061130063512.GA19492@in.ibm.com> <20061130082934.GB29609@elte.hu>
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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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