Re: [PATCH 1/2] signal: simplify deadlock-avoidance in lock_task_sighand()

[Steven Rostedt]

On Mon, 22 Sep 2014 18:44:37 +0200
Oleg Nesterov <oleg@xxxxxxxxxx> wrote:

> __lock_task_sighand() does local_irq_save() to prevent the potential
> deadlock, we can use preempt_disable() with the same effect. And in
> this case we can do preempt_disable/enable + rcu_read_lock/unlock only
> once outside of the main loop and simplify the code. This also shaves
> 112 bytes from signal.o.
> 
> With this patch the main loop runs with preemption disabled, but this
> should be fine because restart is very unlikely: it can only happen if
> we race with de_thread() and ->sighand is shared. And the latter is only
> possible if CLONE_SIGHAND was used without CLONE_THREAD, most probably
> nobody does this nowadays.
> 
> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> ---
>  kernel/signal.c |   31 +++++++++++++------------------
>  1 files changed, 13 insertions(+), 18 deletions(-)
> 
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 8f0876f..61a1f55 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1261,30 +1261,25 @@ struct sighand_struct *__lock_task_sighand(struct task_struct *tsk,
>  					   unsigned long *flags)
>  {
>  	struct sighand_struct *sighand;
> -
> +	/*
> +	 * We are going to do rcu_read_unlock() under spin_lock_irqsave().
> +	 * Make sure we can not be preempted after rcu_read_lock(), see
> +	 * rcu_read_unlock() comment header for details.
> +	 */
> +	preempt_disable();

The sad part is, this is going to break -rt. Or is this something we
can have preempt_disable_nort() with (for the -rt kernel that is). That
is, is -rt susceptible to this deadlock as well?

-- Steve


> +	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;
> -		}
>  
> -		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();
> +	preempt_enable();
>  
>  	return sighand;
>  }

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