linux-kernel - Re: [PATCH 2/3] printk: Add %ptc to safely print a task's comm

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Date:	Tue, 17 May 2011 14:52:30 -0700
From:	Joe Perches <joe@...ches.com>
To:	Jiri Slaby <jirislaby@...il.com>
Cc:	John Stultz <john.stultz@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Michal Nazarewicz <mina86@...a86.com>,
	Andy Whitcroft <apw@...onical.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Subject: Re: [PATCH 2/3] printk: Add %ptc to safely print a task's comm

On Tue, 2011-05-17 at 23:42 +0200, Jiri Slaby wrote:
> On 05/17/2011 10:47 PM, John Stultz wrote:
> > Accessing task->comm requires proper locking. However in the past
> > access to current->comm could be done without locking. This
> > is no longer the case, so all comm access needs to be done
> > while holding the comm_lock.
> > +static noinline_for_stack
> I still fail to see why this should be slowed down by noinlining it.
> Care to explain?

Any vsprintf is slow.

> With my setup, the code below inlined will use 32 bytes of stack. The
> same as %pK case. Uninlined it obviously eats "only" 8 bytes for IP.

The idea is to avoid excess stack consumption for things like:

	struct va_format vaf;

	const char *fmt = "some format with %ptc";

	vaf.fmt = fmt;
	vaf.va = &va_list;

	printk("some format with %pV\n", &vaf);

> > +char *task_comm_string(char *buf, char *end, void *addr,
> > +			 struct printf_spec spec, const char *fmt)
> > +{
> > +	struct task_struct *tsk = addr;
> > +	char *ret;
> > +	unsigned long flags;
> > +
> > +	spin_lock_irqsave(&tsk->comm_lock, flags);
> > +	ret = string(buf, end, tsk->comm, spec);
> > +	spin_unlock_irqrestore(&tsk->comm_lock, flags);
> > +
> > +	return ret;
> > +}

I think it was more of a problem when "4k stacks" was the default
than today, but I think it is still "good form". 

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