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

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Date:   Sun, 9 Apr 2017 12:59:18 +0200
From:   Andreas Mohr <andi@...as.de>
To:     Pavel Machek <pavel@....cz>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Petr Mladek <pmladek@...e.com>, 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 Thu, Apr 06, 2017 at 07:20:52PM +0200, Pavel Machek wrote:
> On Wed 2017-03-29 18:25:07, Sergey Senozhatsky wrote:
> > It's not always possible/safe to wake_up() printk kernel
> > thread. For example, late suspend/early resume may printk()
> > while timekeeping is not initialized yet, so calling into the
> > scheduler may result in recursive warnings.
> > 
> > Another thing to notice is the fact PM at some point
> > freezes user space and kernel threads: freeze_processes()
> > and freeze_kernel_threads(), correspondingly. Thus we need
> > printk() to operate in old mode there and attempt to
> > immediately flush pending kernel message to the console.
> > 
> > This patch adds printk_emergency_begin/on sections.
> > 
> > Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> 
> I don't like this. It is symptom of printk getting much more fragile
> now.

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);

That way it could (hopefully) be ensured that
people could use a consistent "printk" *interface* universally regardless of which
"special" conditions happen to be in place at the moment.
(IOW, keep interface behaviour which is required/expected at user code
definitely isolated from
awkward "implementation aspects" necessity which is currently poisoning user code implementation).

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.

...or so goes the theory.
(quite possibly such thoughts may hit roadblocks e.g. due to locking/atomicity issues)

HTH,

Andreas Mohr

-- 
GNU/Linux. It's not the software that's free, it's you.