linux-kernel - Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

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Message-ID: <20170630125044.GC792@jagdpanzerIV.localdomain>
Date:   Fri, 30 Jun 2017 21:50:44 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     sergey.senozhatsky.work@...il.com, pmladek@...e.com,
        sergey.senozhatsky@...il.com, rostedt@...dmis.org, jack@...e.cz,
        akpm@...ux-foundation.org, peterz@...radead.org, rjw@...ysocki.net,
        ebiederm@...ssion.com, gregkh@...uxfoundation.org, jslaby@...e.com,
        pavel@....cz, andi@...as.de, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

On (06/30/17 21:35), Tetsuo Handa wrote:
> Date: Fri, 30 Jun 2017 21:35:28 +0900
> From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> To: sergey.senozhatsky.work@...il.com
> Cc: pmladek@...e.com, sergey.senozhatsky@...il.com, rostedt@...dmis.org,
>  jack@...e.cz, akpm@...ux-foundation.org, peterz@...radead.org,
>  rjw@...ysocki.net, ebiederm@...ssion.com, gregkh@...uxfoundation.org,
>  jslaby@...e.com, pavel@....cz, andi@...as.de, linux-kernel@...r.kernel.org
> Subject: Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread
> X-Mailer: Winbiff [Version 2.51 PL2]
> 
> Sergey Senozhatsky wrote:
> > On (06/30/17 19:18), Tetsuo Handa wrote:
> > > > I'm still thinking about Steven's proposals; but we will need offloading
> > > > anyways, so the bits we are talking about here are important regardless
> > > > the direction printk design will take, I think.
> > > 
> > > Is there a chance that printk() waits for only data queued by that printk()
> > > call (exception will be printk() from NMI).
> > 
> > hm, I don't think this can be done easily... consider
> > 
> > 	console_lock();
> > 	printk();
> > 	printk();
> > 	...			-> this guys will wait forever. nothing
> > 				   flushes the logbuf.
> > 	printk();
> > 	console_unlock();
> 
> Can't we remove console_lock()/console_unlock() from printk() ?

we can't... well, we can... and there are some ideas but we are years
away from all it becoming reality. console_sem is the giant and major
lock that fbcon, drm, tty and so on are using. please see a sub-thread
starting from here:

https://lists.linuxfoundation.org/pipermail/ksummit-discuss/2017-June/004389.html

> I think that printk() depends on console_unlock() is complicating.

it absolutely is.

	-ss