linux-kernel - Re: [PATCH v10 1/2] printk: Make printk() completely async

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Message-ID: <20160818105629.GE26194@pathway.suse.cz>
Date:	Thu, 18 Aug 2016 12:56:29 +0200
From:	Petr Mladek <pmladek@...e.com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>, Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Jan Kara <jack@...e.com>, Tejun Heo <tj@...nel.org>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Byungchul Park <byungchul.park@....com>, vlevenetz@...sol.com,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v10 1/2] printk: Make printk() completely async

On Thu 2016-08-18 18:51:44, Sergey Senozhatsky wrote:
> On (08/18/16 11:33), Petr Mladek wrote:
> [..]
> > > > 2. Force synchronous printk inside WARN()/BUG() macros.
> > > 
> > > will it help? semaphore up() calls wake_up_process() regardless the context.
> > > not to mention that we still may have spin_dump() enabled.
> > 
> > Good point. That changes my preferences :-)
> > 
> > > 
> > > > 3. Force printk_deferred() inside WARN()/BUG() macros via the per-CPU
> > > >    printk_func.
> > > > 
> > > >    It might be elegant. But we do not want this outside the scheduler
> > > >    code. Therefore we would need special variants of  WARN_*_SCHED()
> > > >    BUG_*_SCHED() macros.
> > 
> > Also we need to make sure that everything will be done on a single CPU
> > as the printk_func is per-CPU variable.
> 
> lib/dump_stack.c dump_stack() disables local IRQs, so we will stay
> on the same CPU.

It is less important to stay of the same CPU if you use
printk_deferred() directly. But it us must to have if you
use it via the printk_func per-CPU variable.

The advantage of the printk_func trick is that it is transparent.
You do not need to modify any existing functions used by WARN()/BUG()
macros.

Best Regards,
Petr