linux-kernel - Re: [PATCH v2 2/3] freezer: do not send a fake signal to a PF

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Message-ID: <45402727.PXDVBuj6zy@vostro.rjw.lan>
Date:	Mon, 25 Feb 2013 00:39:03 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mandeep Singh Baines <msb@...omium.org>,
	Neil Horman <nhorman@...hat.com>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] freezer: do not send a fake signal to a PF_DUMPCORE thread

On Sunday, February 24, 2013 07:36:56 PM Oleg Nesterov wrote:
> A coredumping thread can't be frozen anyway but the fake signal sent
> by freeze_task() can confuse dump_write/wait_for_dump_helpers/etc
> and interrupt the coredump.
> 
> We are going to make the do_coredump() paths freezable but the fake
> TIF_SIGPENDING doesn't help, it only makes sense when we assume that
> the target can return to user-mode and call get_signal_to_deliver().
> 
> Change freeze_task() to check PF_DUMPCORE along with PF_KTHREAD. We
> need to recheck PF_DUMPCORE under ->siglock to avoid the race with
> zap_threads() which can set this flag right before we take the lock.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

> ---
>  kernel/freezer.c |   19 ++++++++++++-------
>  1 files changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/freezer.c b/kernel/freezer.c
> index c38893b..595afab 100644
> --- a/kernel/freezer.c
> +++ b/kernel/freezer.c
> @@ -85,14 +85,21 @@ bool __refrigerator(bool check_kthr_stop)
>  }
>  EXPORT_SYMBOL(__refrigerator);
>  
> -static void fake_signal_wake_up(struct task_struct *p)
> +static bool fake_signal_wake_up(struct task_struct *p)
>  {
>  	unsigned long flags;
> +	bool ret = false;
> +
> +	if (p->flags & (PF_KTHREAD | PF_DUMPCORE))
> +		return ret;
>  
>  	if (lock_task_sighand(p, &flags)) {
> -		signal_wake_up(p, 0);
> +		ret = !(p->flags & PF_DUMPCORE);
> +		if (ret)
> +			signal_wake_up(p, 0);
>  		unlock_task_sighand(p, &flags);
>  	}
> +	return ret;
>  }
>  
>  /**
> @@ -100,8 +107,8 @@ static void fake_signal_wake_up(struct task_struct *p)
>   * @p: task to send the request to
>   *
>   * If @p is freezing, the freeze request is sent either by sending a fake
> - * signal (if it's not a kernel thread) or waking it up (if it's a kernel
> - * thread).
> + * signal (if it's not a kernel thread or a coredumping thread) or waking
> + * it up otherwise.
>   *
>   * RETURNS:
>   * %false, if @p is not freezing or already frozen; %true, otherwise
> @@ -116,9 +123,7 @@ bool freeze_task(struct task_struct *p)
>  		return false;
>  	}
>  
> -	if (!(p->flags & PF_KTHREAD))
> -		fake_signal_wake_up(p);
> -	else
> +	if (!fake_signal_wake_up(p))
>  		wake_up_state(p, TASK_INTERRUPTIBLE);
>  
>  	spin_unlock_irqrestore(&freezer_lock, flags);
> 
-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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