linux-kernel - Re: [PATCH v4 2/3] printk: set may_schedule for some of console

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Date:	Wed, 17 Feb 2016 00:45:17 +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>
Subject: Re: [PATCH v4 2/3] printk: set may_schedule for some of
 console_trylock callers

On (02/16/16 15:43), Petr Mladek wrote:
> On Sat 2016-02-13 03:37:11, Sergey Senozhatsky wrote:
> > console_unlock() allows to cond_resched() if its caller has
> > set `console_may_schedule' to 1, since
> > 'commit 8d91f8b15361 ("printk: do cond_resched() between lines while
> > outputting to consoles")'.
> > 
> > The rules are:
> > -- console_lock() always sets `console_may_schedule' to 1
> > -- console_trylock() always sets `console_may_schedule' to 0
> > 
> > However, console_trylock() callers (among them is printk()) do
> > not always call printk() from atomic contexts, and some of them
> > can cond_resched() in console_unlock(), so console_trylock()
> > can set `console_may_schedule' to 1 for such processes.
> > 
> > For !CONFIG_PREEMPT_COUNT kernels, however, console_trylock()
> > always sets `console_may_schedule' to 0.
> > 
> > It's possible to drop explicit preempt_disable()/preempt_enable()
> > in vprintk_emit(), because console_unlock() and console_trylock()
> > are now smart enough:
> > a) console_unlock() does not cond_resched() when it's unsafe
> >   (console_trylock() takes care of that)
> > b) console_unlock() does can_use_console() check.
> > 
> > Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> 
> It looks safe after all.
> 
> Reviewed-by: Petr Mladek <pmladek@...e.com>
> 

thanks.

	-ss