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Subject: Re: [RFC][PATCH 4/7] lockdep: Seperate lock ids for read/write
 acquires
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Ingo Molnar <mingo@elte.hu>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Gautham R Shenoy <gautshen@cmi.ac.in>
In-Reply-To: <20110417095507.123045423@chello.nl>
References: <20110417094505.865828233@chello.nl>
	 <20110417095507.123045423@chello.nl>
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Date: Tue, 19 Apr 2011 00:07:00 +0200
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On Sun, 2011-04-17 at 11:45 +0200, Peter Zijlstra wrote:
> In order to support recursive read locks we need to support the
> previously mentioned lock state conflict matrix:
> 
>  Conflicting_states(WRITE):             RECURSIVE_READ | READ | WRITE
>  Conflicting_states(READ):                               READ | WRITE
>  Conflicting_states(RECURSIVE_READ):                            WRITE
> 
> Since this introduces asymmetry between recursive read and write, we
> need to split the lock dependency chains such that we can traverse
> WRITE chains without observing RECURSIVE_READ|READ chains. 

So while we split off the WRITE chain from the RECURSIVE_READ|READ
chains, shouldn't we split it in three, because the READ conflict state
only has READ|WRITE, not RECURSIVE_READ.

Therefore a RECURSIVE_READ dependency in the READ chain could throw the
regular READ cycle detector, no?


 A(r) -> B(r) -> C(rr)

 C(w) -> B(r)

would close the cycle and report a problem, but doesn't match the
conflict states.