linux-kernel - Re: [RFC][PATCH 4/7] lockdep: Seperate lock ids for read/write acquires

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Message-ID: <1303164420.32491.908.camel@twins>
Date:	Tue, 19 Apr 2011 00:07:00 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Gautham R Shenoy <gautshen@....ac.in>
Subject: Re: [RFC][PATCH 4/7] lockdep: Seperate lock ids for read/write
 acquires

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.
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