linux-kernel - Re: [RFC][PATCHv2 4/8] pm: switch to printk.emergency mode in unsafe places

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Date:   Mon, 10 Apr 2017 23:38:17 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Andreas Mohr <andi@...as.de>, Petr Mladek <pmladek@...e.com>
Cc:     Pavel Machek <pavel@....cz>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>, Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Eric Biederman <ebiederm@...ssion.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Len Brown <len.brown@...el.com>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC][PATCHv2 4/8] pm: switch to printk.emergency mode in unsafe
 places

On (04/10/17 14:20), Petr Mladek wrote:
[..]
> > Sergey has mentioned it already:
> > "at some point freezes user space and kernel threads".
> > Well, this is the action which is *itself* causing thoroughly disrupting consequences,
> > which I'd think thus ought to be responsible to
> > ensure *itself* that all resulting consequences actually can be dealt with properly,
> > rather than having
> > weird *completely-unrelated-dependency* crap
> > ("there happens to be some functionality called printk, and we need to bend it,
> > since we need to bend it, since otherwise it would not be bent" - ahem...)
> > leak into ("layer violation" keyword)
> > pm handling implementation specifics.
> > IOW, I would think that for any relevant kthread use in API user code,
> > such code ought to be able to
> > register kthread-API-provided callbacks (observer pattern, or whatever)
> > where the (back to current case:) printk kthread would then be able to
> > *implicitly*/*invisibly* switch the entire printk operation interface
> > (e.g. via a global interface struct) to
> > the "dumb"/"safe" fallback variant.
> > Potential interface: kthread_notify(callback_func, kthread_notification_type);
> 
> Interesting idea. The power management area probably can be solved
> by the existing notifiers framework.

good idea indeed.

wish we also had kexec and sysrq notifiers :) there is a
`panic_notifier_list', but that's not exactly what we need.

[..]
> > Put differently,
> > handling preferrably ought to get consistently adapted (i.e., switched) *centrally*,
> > rather than
> > requiring weird helpers (printk_emergency_X()) at all user code sites.
> 
> Note that there already all many printk/console related "hacks"
> in sensitive code paths. For example, see the use of
> pm_prepare_console(), suspend_console(), console_level.

yep. I wonder if some of those can be moved to printk pm notifiers.
but that's out of the scope of this patch set.

	-ss