linux-kernel - Re: [PATCH tip/core/urgent 1/3] rcu: fix lockdep splat in wake

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Date:	Wed, 16 Jun 2010 08:23:11 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, Daniel J Blueman <daniel.blueman@...il.com>
Subject: Re: [PATCH tip/core/urgent 1/3] rcu: fix lockdep splat in
 wake_affine()

On Tue, 2010-06-15 at 15:29 -0700, Paul E. McKenney wrote:
> From: Daniel J Blueman <daniel.blueman@...il.com>
> 
> With 2.6.35-rc1 and your patch in the context below, we still see
> "include/linux/cgroup.h:534 invoked rcu_dereference_check() without
> protection!", so need this additional patch:
> 
> Acquire read-side RCU lock around task_group() calls, addressing
> "include/linux/cgroup.h:534 invoked rcu_dereference_check() without
> protection!" warning.

Uhm,. this is all just slapping in rcu_read_lock() to make the warning
go away, without explanation of what and why.

Its not obvious what the races is, nor how its handled.

> Signed-off-by: Daniel J Blueman <daniel.blueman@...il.com>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
>  kernel/sched_fair.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index eed35ed..ca56133 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -1240,6 +1240,7 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
>  	 * effect of the currently running task from the load
>  	 * of the current CPU:
>  	 */
> +	rcu_read_lock();
>  	if (sync) {
>  		tg = task_group(current);
>  		weight = current->se.load.weight;
> @@ -1249,6 +1250,7 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
>  	}
>  
>  	tg = task_group(p);
> +	rcu_read_unlock();
>  	weight = p->se.load.weight;
>  
>  	/*


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