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Subject: Re: [PATCH 2/2] lockdep: Don't only check recursive read locks
 once in a sequence
From: Peter Zijlstra <peterz@infradead.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>, Ingo Molnar <mingo@elte.hu>,
       LKML <linux-kernel@vger.kernel.org>,
       Thomas Gleixner <tglx@linutronix.de>, Ming Lei <tom.leiming@gmail.com>
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Date: Thu, 19 Nov 2009 18:13:48 +0100
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On Thu, 2009-11-19 at 16:55 +0100, Frederic Weisbecker wrote:
> And btw I
> don't understand why rwsem read locks are not considered as
> recursive in lockdep. 

Because they're not.

rwsems are FIFO fair, so a double read can deadlock when there's a
pending writer inbetween.