linux-kernel - Re: lock_task_sighand() && rcu

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Date:	Mon, 5 May 2014 18:47:20 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: lock_task_sighand() && rcu_boost()

On 05/05, Paul E. McKenney wrote:
>
> Does the patch below cover it?

Yes, thanks.

Acked-by: Oleg Nesterov <oleg@...hat.com>

> signal: Explain local_irq_save() call
> 
> The explicit local_irq_save() in __lock_task_sighand() is needed to avoid
> a potential deadlock condition, as noted in a841796f11c90d53 (signal:
> align __lock_task_sighand() irq disabling and RCU).  However, someone
> reading the code might be forgiven for concluding that this separate
> local_irq_save() was completely unnecessary.  This commit therefore adds
> a comment referencing the shiny new block comment on rcu_read_unlock().
> 
> Reported-by: Oleg Nesterov <oleg@...hat.com>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 6ea13c09ae56..513e8c252aa4 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1288,6 +1288,10 @@ struct sighand_struct *__lock_task_sighand(struct task_struct *tsk,
>  	struct sighand_struct *sighand;
>  
>  	for (;;) {
> +		/*
> +		 * Disable interrupts early to avoid deadlocks.
> +		 * See rcu_read_unlock comment header for details.
> +		 */
>  		local_irq_save(*flags);
>  		rcu_read_lock();
>  		sighand = rcu_dereference(tsk->sighand);
> 

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