linux-kernel - Re: [patch 2/4] tree rcu: Add debug RCU head option

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Date:	Tue, 06 Oct 2009 18:17:51 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [patch 2/4] tree rcu: Add debug RCU head option

Mathieu Desnoyers a écrit :
> * Eric Dumazet (eric.dumazet@...il.com) wrote:
>> Mathieu Desnoyers a écrit :
>>> Poisoning the rcu_head callback list. Only for rcu tree for now.
>>>
>>> Helps finding racy users of call_rcu(), which results in hangs because list
>>> entries are overwritten and/or skipped. Using the lower bit to poison because
>>> include/net/dst.h __pad_to_align_refcnt complains when struct rcu_head grows.
>>>
>> I see :)
>>
> 
> I don't know if there is an easy fix for __pad_to_align_refcnt ?


This check was added to make sure some tbench regression was not added if
dst->__refcnt was moved around, I am sure you dont care about tbench being 10% slower,
do you ?


diff --git a/include/net/dst.h b/include/net/dst.h
index 5a900dd..b8fba74 100644
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -154,7 +154,9 @@ static inline void dst_hold(struct dst_entry * dst)
 	 * If your kernel compilation stops here, please check
 	 * __pad_to_align_refcnt declaration in struct dst_entry
 	 */
+#if !defined(DEBUG_RCU_HEAD)
 	BUILD_BUG_ON(offsetof(struct dst_entry, __refcnt) & 63);
+#endif
 	atomic_inc(&dst->__refcnt);
 }
 

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