linux-kernel - Re: [RFC][PATCH v3 4/4] printk: set may_schedule for some of console

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Date:	Fri, 12 Feb 2016 00:02:17 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.com>, Tejun Heo <tj@...nel.org>,
	Kyle McMartin <kyle@...nel.org>,
	Dave Jones <davej@...emonkey.org.uk>,
	Calvin Owens <calvinowens@...com>,
	linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC][PATCH v3 4/4] printk: set may_schedule for some of
 console_trylock callers

Hello Petr,

On (02/11/16 15:41), Petr Mladek wrote:
[..]
> > +	console_may_schedule = !oops_in_progress &&
> > +			preemptible() &&
> > +			!rcu_preempt_depth();
> >  	return 1;
> 
> We discussed this a lot but I am still a bit nervous ;-)

sure, no prob :-)

> Avoid scheduling when oops_in_progress makes sense.
> 
> preemptible() takes care of preemption and IRQ contexts.
> The comment above explains that it is safe to use here.
> 
> The check for rcu_preempt_depth() makes sense. But is it
> safe, please?
> 
> rcu_preempt_depth() returns 0 if CONFIG_PREEMPT_RCU is not
> enabled. It means that you are not able to detect RCU read
> section and it might cause problems.

well, I believe it's ok. __rcu_read_lock() for CONFIG_PREEMPT_RCU
does current->rcu_read_lock_nesting++, so rcu_preempt_depth() works
as expected. otherwise, for !CONFIG_PREEMPT_RCU kernel,
__rcu_read_lock() does

	if (IS_ENABLED(CONFIG_PREEMPT_COUNT))
		preempt_disable()


- if we run "CONFIG_PREEMPT_RCU" then rcu_preempt_depth()
  works here.

- if we run "!CONFIG_PREEMPT_RCU && CONFIG_PREEMPT_COUNT"
  then preemptible() works for us

- if we run "!CONFIG_PREEMPT_RCU && !CONFIG_PREEMPT_COUNT"
  then preemptible() is always 0.

> I rather add Paul into CC.

thanks.

	-ss