linux-kernel - Re: Weird rcu lockdep warning

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Message-ID: <4BC537C9.8050600@cn.fujitsu.com>
Date:	Wed, 14 Apr 2010 11:34:33 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	David Miller <davem@...emloft.net>, fweisbec@...il.com,
	a.p.zijlstra@...llo.nl, mingo@...e.hu, sparclinux@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Weird rcu lockdep warning

Paul E. McKenney wrote:
> On Tue, Apr 13, 2010 at 05:13:06PM -0700, David Miller wrote:
>> From: Frederic Weisbecker <fweisbec@...il.com>
>> Date: Wed, 14 Apr 2010 02:02:27 +0200
>>
>>> I just have a guess though....
>>> This seems to always happen from NMI path, and lockdep is disabled on NMI.
>>> I suspect the lock_acquire() performed by rcu_read_lock() is just ignored
>>> and then the rcu_read_lock_held() check has the wrong result...
>> Yeah, I bet that's it too.
>>
>> lock_is_held() can't return anything meaningful while lockdep is
>> disabled, which it is during NMIs.
> 
> Ah!  So I just need to add a "current->lockdep_recursion"
> check to debug_lockdep_rcu_enabled().  And move the function to
> kernel/rcutree_plugin.h to avoid #include hell.
> 
> See below for (untested) patch.
> 
> 						Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
>  include/linux/rcupdate.h |    5 +----
>  kernel/rcutree_plugin.h  |   11 +++++++++++
>  2 files changed, 12 insertions(+), 4 deletions(-)
> 
> commit 304d8da6cd791a81ce3164f867e1b3ef4f9af1d1
> Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Date:   Tue Apr 13 18:45:51 2010 -0700
> 
>     rcu: Make RCU lockdep check the lockdep_recursion variable
>     
>     The lockdep facility temporarily disables lockdep checking by incrementing
>     the current->lockdep_recursion variable.  Such disabling happens in NMIs
>     and in other situations where lockdep might expect to recurse on itself.
>     This patch therefore checks current->lockdep_recursion, disabling RCU
>     lockdep splats when this variable is non-zero.
>     
>     Reported-by: Frederic Weisbecker <fweisbec@...il.com>
>     Reported-by: David Miller <davem@...emloft.net>
>     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 9f1ddfe..07db2fe 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -101,10 +101,7 @@ extern struct lockdep_map rcu_sched_lock_map;
>  # define rcu_read_release_sched() \
>  		lock_release(&rcu_sched_lock_map, 1, _THIS_IP_)
>  
> -static inline int debug_lockdep_rcu_enabled(void)
> -{
> -	return likely(rcu_scheduler_active && debug_locks);
> -}
> +extern int debug_lockdep_rcu_enabled(void);
>  
>  /**
>   * rcu_read_lock_held - might we be in RCU read-side critical section?
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index 79b53bd..2169abe 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -1067,3 +1067,14 @@ static void rcu_needs_cpu_flush(void)
>  }
>  
>  #endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */
> +
> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> +
> +int debug_lockdep_rcu_enabled(void)
> +{
> +	return likely(rcu_scheduler_active &&
> +		      debug_locks &&
> +		      current->lockdep_recursion == 0);
> +}
> +

Looks good to me too, but I think
'likely' is needless since the function is not inline.

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