Re: [PATCH tip/core/rcu 03/17] signal: Explain local_irq_save() call

[Lai Jiangshan]

On 07/08/2014 06:38 AM, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> ---
>  kernel/signal.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/signal.c b/kernel/signal.c
> index a4077e90f19f..46161e744760 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1263,6 +1263,10 @@ 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.
> +		 */

A pair of brackets are missing here: rcu_read_unlock()
after that, please add:

Reviewed-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>


It reminds me that I should keep my effort to solve the deadlock
problem where rcu_read_unlock() is overlapped with schedule locks.

>  		local_irq_save(*flags);
>  		rcu_read_lock();
>  		sighand = rcu_dereference(tsk->sighand);

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