linux-kernel - Re: [GIT PULL] printk for 5.19

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Date:   Thu, 26 May 2022 14:09:25 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Jan Kara <jack@...e.cz>, Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] printk for 5.19

On Thu 2022-05-26 00:08:46, John Ogness wrote:
> On 2022-05-25, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> > On Mon, May 23, 2022 at 6:21 AM Petr Mladek <pmladek@...e.com> wrote:
> >> There are situations when the kthreads are either not available or
> >> not reliable, for example, early boot, suspend, or panic. In these
> >> situations, printk() uses the legacy mode and tries to handle
> >> consoles immediately.
> >
> > Looking through the commits, I don't see how that "printk: wake up all
> > waiters" makes any sense at all.
> >
> > It *ALREADY* woke up all waiters as far as I can see.
> >
> > Doing a wake_up_interruptible() will stop waking things up only when
> > it hits a *exclusive* waiter, and as far as I can tell, there are no
> > exclusive waiters there.
> 
> You are correct. @log_wait never has exclusive waiters. I will post a
> patch to revert the change in question.

John, Yes, please do so.

> > But when I see something really core like printk() get confused and
> > mis-understand basic wait queue behavior, that makes me go "This is
> > WRONG".

Linus, thanks a lot for catching and explaining the problem.

Yeah, it is a shame. I have met code with exclusive waiters only once,
8 years ago and completely forgot about it. I was curious how it
worked with more log waiters but I put it into TODO list. The new
code looked safe and I was too focused on finding potential problems.

Best Regards,
Petr