linux-kernel - Re: INFO: possible circular locking dependency detected

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Date:	Thu, 14 Jul 2011 13:46:53 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dipankar Sarma <dipankar@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: INFO: possible circular locking dependency detected

egad! Looking at this code more, there's nothing keeping
t->rcu_read_unlock_special safe! If it can be modified by the kthread,
and current, then we must use atomic operations or modify under lock.
Otherwise the old read/modify/write can corrupt it.

		t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;

is done before the lock is taken in rcu_read_unlock_special. If the
kthread is running rcu_boost() then its code:

	t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BOOSTED;

Can even negate the removing of the RCU_READ_UNLOCK_BLOCKED!

-- Steve

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