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 14:11:35 +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

On (02/11/16 17:10), Petr Mladek wrote:
[..]
> > 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 feel convinced. But we should somehow document it. I think how
> to do it effectively. I think that the following text would help
> me if I read it:
> 
> 	/*
> 	 * Safe context for rescheduling is detected only when
> 	 * PREEMPT_COUNT is enabled. preemptible() always returns
> 	 * false otherwise.
> 	 *
> 	 * RCU read sections must be detected separately. They
> 	 * have a separate preemption counter when PREEMPT_RCU
> 	 * is enabled.
> 	 */
> 
> I wanted to highlight why exactly the check returns 0 in !PREEMPT_COUNT
> kernel. I missed this a bit in you original comment. But feel free
> to change it as you like.

good point. thanks! will re-spin the patch set later today,
have no reliable internet connection at the moment.

	-ss