linux-kernel - Re: [RFC PATCH v1 00/25] printk: new implementation

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Date:   Wed, 13 Mar 2019 18:27:59 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Sebastian Siewior <bigeasy@...utronix.de>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH v1 00/25] printk: new implementation

On (03/13/19 09:40), Sebastian Siewior wrote:
> On 2019-03-13 09:19:32 [+0100], John Ogness wrote:
> > recursive situation. As you are pointing out, the notification/wake
> > component of printk_safe will still be needed. I will leave that (small)
> > part in printk_safe.c.
> 
> Does this mean we keep irq_work part or we bury it and solve it by other
> means?

That's a very good question. Because if we add console_trylock()
to printk(), then we can't invoke ->atomic() consoles when console_sem
is already locked, because one of the registered drivers is currently
being modified by a 3rd party and printk(), thus, must stay away.
Once that modification will be done console_unlock() will print all
pending messages. This is current design. And this conflicts with the
whole idea of ->atomic() consoles.

So may be we need a whole new scheme in this department as well.

For instance [*and this is completely untested idea* !!!]

*May be* we can take a closer look and find cases when ->atomic
consoles don't really depend on console_sem. And *may be* we can
split the console drivers list and somehow forbid removal and
modification (ioctl) of ->atomic consoles under us. Assuming that
this is doable we then can start iterating ->atomic consoles list
with unlocked console_sem.
Non->atomic consoles or consoles which depend on console_sem (VT,
fbcon and so on) will stay in another list, and we will take
console_sem before we iterate that list and invoke those drivers.

One more time - a completely random thought.

	-ss