linux-kernel - Re: [PATCH] show message when exceeded rlimit of pending signals

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Message-Id: <20091030143333.414ea29c.akpm@linux-foundation.org>
Date:	Fri, 30 Oct 2009 14:33:33 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Naohiro Ooiwa <nooiwa@...aclelinux.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>,
	roland@...hat.com, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, oleg@...hat.com
Subject: Re: [PATCH] show message when exceeded rlimit of pending signals

On Fri, 30 Oct 2009 20:36:31 +0900
Naohiro Ooiwa <nooiwa@...aclelinux.com> wrote:

> Hi Ingo,
> 
> I wrote proper changelog entry.
> And I resent the patch. I added KERN_INFO to printk.
> 
> 
> 
> When the system has too many timers or too many aggregate
> queued signals, the EAGAIN error is returned to application
> from kernel, including timer_create().
> It means that exceeded limit of pending signals at all.
> But we can't imagine it.
> 
> This patch show the message when reached limit of pending signals.
> If you see this message and your system behaved unexpectedly,
> you can run following command.
>    # ulimit -i unlimited
> 
> With help from Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>.
> 
>
> ...
>
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 6705320..50e10dc 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -41,6 +41,8 @@
> 
>  static struct kmem_cache *sigqueue_cachep;
> 
> +int print_fatal_signals __read_mostly;
> +
>  static void __user *sig_handler(struct task_struct *t, int sig)
>  {
>  	return t->sighand->action[sig - 1].sa.sa_handler;
> @@ -188,6 +190,14 @@ int next_signal(struct sigpending *pending, sigset_t *mask)
>  	return sig;
>  }
> 
> +static void show_reach_rlimit_sigpending(void)
> +{
> +	if (!printk_ratelimit())
> +		return;

printk_ratelimit() is a bad thing and we should be working toward
removing it altogether, not adding new callers.

Because it uses global state.  So if subsystem A is trying to generate
lots of printk's, subsystem B's important message might get
accidentally suppressed.

It's better to use DEFINE_RATELIMIT_STATE() and __ratelimit() directly.


> +	printk(KERN_INFO "%s/%d: reached the limit of pending signals.\n",
> +				current->comm, current->pid);

I suggest that this be

	"reached RLIMIT_SIGPENDING"

because RLIMIT_SIGPENDING is a well-understood term and concept.

>  static void print_fatal_signal(struct pt_regs *regs, int signr)
>  {
> -	printk("%s/%d: potentially unexpected fatal signal %d.\n",
> +	printk(KERN_INFO "%s/%d: potentially unexpected fatal signal %d.\n",
>  		current->comm, task_pid_nr(current), signr);
> 

This is an unchangelogged, unrelated, non-backward-compatible
user-visible change.  For some people, their machine which used to
print this warning will mysteriously stop doing so when they upgrade
their kernels.

That doesn't mean that we shouldn't make the change.  But we should
have a think about it and we shouldn't hide changes of this nature
inside some other patch like this.

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