linux-kernel - Re: [PATCH] kernel/signal: Remove no longer required irqsave/restore

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Date:   Fri, 04 May 2018 11:59:08 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>
Subject: Re: [PATCH] kernel/signal: Remove no longer required irqsave/restore

Sebastian Andrzej Siewior <bigeasy@...utronix.de> writes:

> From: Anna-Maria Gleixner <anna-maria@...utronix.de>
>
> Commit a841796f11c9 ("signal: align __lock_task_sighand() irq disabling and
> RCU") introduced a rcu read side critical section with interrupts
> disabled. The changelog suggested that a better long-term fix would be "to
> make rt_mutex_unlock() disable irqs when acquiring the rt_mutex structure's
> ->wait_lock".
>
> This long-term fix has been made in commit 4abf91047cf ("rtmutex: Make >
> wait_lock irq safe") for different reason.

Which tree has this change been made in?  I am not finding the commit
you mention above in Linus's tree.

> Therefore revert commit a841796f11c9 ("signal: align >
> __lock_task_sighand() irq disabling and RCU") as the interrupt disable
> dance is not longer required.

Which tree is change aimed at?

Eric

> Signed-off-by: Anna-Maria Gleixner <anna-maria@...utronix.de>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  kernel/signal.c | 24 +++++++-----------------
>  1 file changed, 7 insertions(+), 17 deletions(-)
>
> diff --git a/kernel/signal.c b/kernel/signal.c
> index c6e4c83dc090..16b87c54d027 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1244,19 +1244,12 @@ struct sighand_struct *__lock_task_sighand(struct task_struct *tsk,
>  {
>  	struct sighand_struct *sighand;
>  
> +	rcu_read_lock();
>  	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);
> -		if (unlikely(sighand == NULL)) {
> -			rcu_read_unlock();
> -			local_irq_restore(*flags);
> +		if (unlikely(sighand == NULL))
>  			break;
> -		}
> +
>  		/*
>  		 * This sighand can be already freed and even reused, but
>  		 * we rely on SLAB_TYPESAFE_BY_RCU and sighand_ctor() which
> @@ -1268,15 +1261,12 @@ struct sighand_struct *__lock_task_sighand(struct task_struct *tsk,
>  		 * __exit_signal(). In the latter case the next iteration
>  		 * must see ->sighand == NULL.
>  		 */
> -		spin_lock(&sighand->siglock);
> -		if (likely(sighand == tsk->sighand)) {
> -			rcu_read_unlock();
> +		spin_lock_irqsave(&sighand->siglock, *flags);
> +		if (likely(sighand == tsk->sighand))
>  			break;
> -		}
> -		spin_unlock(&sighand->siglock);
> -		rcu_read_unlock();
> -		local_irq_restore(*flags);
> +		spin_unlock_irqrestore(&sighand->siglock, *flags);
>  	}
> +	rcu_read_unlock();
>  
>  	return sighand;
>  }