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

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Message-ID: <20091006163500.GC15904@Krystal>
Date:	Tue, 6 Oct 2009 12:35:00 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Eric Dumazet <eric.dumazet@...il.com>
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

* Eric Dumazet (eric.dumazet@...il.com) wrote:
> 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 ?
> 

Indeed, I absolutely don't care when I'm doing debugging :)

Will merge, and add a "struct rcu_head *debug" field in struct rcu_head.

Thanks !

Mathieu

> 
> 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);
>  }
>  
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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