linux-kernel - Re: [PATCH] signals: Generate warning when flush_signals() is called from non-kthread context

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Message-ID: <20150502183001.07eae212@notabene.brown>
Date:	Sat, 2 May 2015 18:30:01 +1000
From:	NeilBrown <neilb@...e.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Evgeniy Polyakov <zbr@...emap.net>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	Alex Williamson <alex.williamson@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH] signals: Generate warning when flush_signals() is
 called from non-kthread context

On Fri, 1 May 2015 21:38:13 +0200 Ingo Molnar <mingo@...nel.org> wrote:


>   drivers/md/md.c
>   drivers/md/raid1.c
>   drivers/md/raid5.c
> 
> Hm, so I'm not super sure about the flush_signals() in 
> raid1.c:make_request() AFAICS we can do direct RAID1 writes in 
> raid1_unplug(). That looks unsafe ... I've Cc:-ed Neil.
> 
> raid5.c seems safe: raid5_unplug() doesn't create requests directly, 
> leaves it all for the mddev kthread.

Both raid1.c and raid5.c call flush_signals() in the make_request function
(in unusual circumstances).
I wanted a uninterruptible wait which didn't add to load-average.  That
approach works in kernel threads...

All the calls in md.c are in a kernel thread so safe, but I'd rather have an
explicit "uninterruptible, but no load-average" wait....

I should  probably change the make_request code to queue the request
somewhere rather than wait for it to be serviceable.

I'll look into that...



> In any case, it seems to me that the patch below would be justified? 
> Totally untested and so. __flush_signals() not affected.

Fine by me - does seem justified.

Thanks,
NeilBrown


> 
> Thanks,
> 
> 	Ingo
> 
> ---
>  kernel/signal.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/signal.c b/kernel/signal.c
> index d51c5ddd855c..100e30afe5d2 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -427,6 +427,10 @@ void flush_signals(struct task_struct *t)
>  {
>  	unsigned long flags;
>  
> +	/* Only kthreads are allowed to destroy signals: */
> +	if (WARN_ON_ONCE(!(current->flags & PF_KTHREAD)))
> +		return;
> +
>  	spin_lock_irqsave(&t->sighand->siglock, flags);
>  	__flush_signals(t);
>  	spin_unlock_irqrestore(&t->sighand->siglock, flags);
> --
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