Re: __update_max_tr: rcu_read_lock() used illegally while idle!

From: Steven Rostedt <rostedt@goodmis.org>
To: paulmck@linux.vnet.ibm.com
Cc: Fengguang Wu <fengguang.wu@intel.com>,
	Steven Rostedt <srostedt@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	David Howells <dhowells@redhat.com>
Subject: Re: __update_max_tr: rcu_read_lock() used illegally while idle!
Date: Tue, 31 Jul 2012 10:51:51 -0400	[thread overview]
Message-ID: <1343746311.27983.52.camel@gandalf.stny.rr.com> (raw)
In-Reply-To: <20120731144453.GB2422@linux.vnet.ibm.com>

On Tue, 2012-07-31 at 07:44 -0700, Paul E. McKenney wrote:

> > Found it (and Cc'd David).
> > 
> > In __update_max_tr() we have:
> > 
> > 	max_data = task_uid(tsk);
> > 
> > where task_uid() is:
> > 
> > #define task_uid(task)		(task_cred_xxx((task), uid))
> > 
> > #define task_cred_xxx(task, xxx)			\
> > ({							\
> > 	__typeof__(((struct cred *)NULL)->xxx) ___val;	\
> > 	rcu_read_lock();				\
> > 	___val = __task_cred((task))->xxx;		\
> > 	rcu_read_unlock();				\
> > 	___val;						\
> > })
> > 
> > The __update_max_tr() is called at every location interrupts are enabled
> > (and a max time is discovered). But now this can include places that
> > rcu_read_lock can not be called, I'm not sure how to handle this. Is
> > there a non rcu way to get a tasks uid?
> 
> OK, I will bite.  How about using something like RCU_NONIDLE(), either
> directly or open-coded, to make it a legal call site?

OK, then something like:

	RCU_NONIDLE(max_data = task_uid(tsk));

would work when called normally or with idle?

-- Steve