linux-kernel - Re: [PATCH] notifier atomic call chain notrace

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Message-ID: <20091119000213.GE6683@linux.vnet.ibm.com>
Date:	Wed, 18 Nov 2009 16:02:13 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Jason Baron <jbaron@...hat.com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, hpa@...or.com, tglx@...utronix.de,
	rostedt@...dmis.org, andi@...stfloor.org, roland@...hat.com,
	rth@...hat.com, mhiramat@...hat.com
Subject: Re: [PATCH] notifier atomic call chain notrace

On Wed, Nov 18, 2009 at 06:38:15PM -0500, Mathieu Desnoyers wrote:
> * Jason Baron (jbaron@...hat.com) wrote:
> > Note that this is conditional on gcc >= 4.5. Also there is a !lockdep
> > condition. This is b/c the die_notifier takes an rcu_read_lock() on the
> > int3 trap, which then causes another one etc. Since, we aren't going to be
> > installing removing the handler, the rcu_read_lock() could be avoided for this
> > case with some code restructuring.
> > 
> [snip]
> 
> Would the following patch help ? I use it in the LTTng tree to alleviate
> this problem.
> 
> 
> notifier atomic call chain notrace
> 
> In LTTng, being able to use the atomic notifier from cpu idle entry to
> ensure the tracer flush the last events in the current subbuffer
> requires the rcu read-side to be marked "notrace", otherwise it can end
> up calling back into lockdep and the tracer.
> 
> Also apply to the the die notifier.

Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
> CC: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> CC: Jason Baron <jbaron@...hat.com>
> CC: mingo@...e.hu
> ---
>  kernel/notifier.c |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> Index: linux-2.6-lttng/kernel/notifier.c
> ===================================================================
> --- linux-2.6-lttng.orig/kernel/notifier.c	2009-11-12 17:58:56.000000000 -0500
> +++ linux-2.6-lttng/kernel/notifier.c	2009-11-12 18:03:28.000000000 -0500
> @@ -148,7 +148,7 @@ int atomic_notifier_chain_unregister(str
>  	spin_lock_irqsave(&nh->lock, flags);
>  	ret = notifier_chain_unregister(&nh->head, n);
>  	spin_unlock_irqrestore(&nh->lock, flags);
> -	synchronize_rcu();
> +	synchronize_sched();
>  	return ret;
>  }
>  EXPORT_SYMBOL_GPL(atomic_notifier_chain_unregister);
> @@ -178,9 +178,9 @@ int __kprobes __atomic_notifier_call_cha
>  {
>  	int ret;
> 
> -	rcu_read_lock();
> +	rcu_read_lock_sched_notrace();
>  	ret = notifier_call_chain(&nh->head, val, v, nr_to_call, nr_calls);
> -	rcu_read_unlock();
> +	rcu_read_unlock_sched_notrace();
>  	return ret;
>  }
>  EXPORT_SYMBOL_GPL(__atomic_notifier_call_chain);
> 
> -- 
> Mathieu Desnoyers
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
> --
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