linux-kernel - Re: [RFC patch 2/3] tree/tiny rcu: Add debug RCU head objects (v3)

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Message-ID: <20100319221000.GA5276@x200>
Date:	Sat, 20 Mar 2010 00:10:00 +0200
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
	laijs@...fujitsu.com, dipankar@...ibm.com, josh@...htriplett.org,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com
Subject: Re: [RFC patch 2/3] tree/tiny rcu: Add debug RCU head objects (v3)

On Fri, Mar 19, 2010 at 04:47:41PM -0400, Mathieu Desnoyers wrote:
> Helps finding racy users of call_rcu(), which results in hangs because list
> entries are overwritten and/or skipped.
> 
> This new patch version is based on the debugobjects with the newly introduced
> "active state" tracker.
> 
> Non-initialized entries are all considered as "statically initialized". An
> activation fixup (triggered by call_rcu()) takes care of performing the debug
> object initialization without issuing any warning. Since we cannot increase the
> size of struct rcu_head, I don't see much room to put an identifier for
> statically initialized rcu_head structures. So for now, we have to live without
> "activation without explicit init" detection. But the main purpose of this debug
> option is to detect double-activations (double call_rcu() use of a rcu_head
> before the callback is executed), which is correctly addressed here.
> 
> This also detects potential internal RCU callback corruption, which would cause
> the callbacks to be executed twice.

Is this useful?

Basic usage is so there no double call_rcu():

	if (atomic_dec_and_test())
		call_rcu()
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