linux-kernel - Re: linux.git: printk() problem

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Date:   Mon, 24 Oct 2016 19:06:13 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Joe Perches <joe@...ches.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Petr Mladek <pmladek@...e.cz>, Tejun Heo <tj@...nel.org>,
        Calvin Owens <calvinowens@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: linux.git: printk() problem

On Mon, Oct 24, 2016 at 6:55 PM, Sergey Senozhatsky
<sergey.senozhatsky.work@...il.com> wrote:
>
> I think cont_flush() should grab the logbuf_lock lock, because
> it does log_store() and touches the cont.len. so something like
> this perhaps

Absolutely. Good catch.

>> -     if (wake_klogd)
>> +     if (wake_klogd || cont.len)
>                         ^^^^^^^^^^
> this _technically_ can result in additional spurious wakeups - cont.len
> check is done outside of console_sem && logbuf_lock - but I don't think
> this is a huge problem.

Yes. We could easily just do it inside the spinlock and make it part
of the wake_klogd logic, but as you say, it doesn't actually matter.

And yes, it would be lovely if people who use the network console
(which I think is the only user of the whole extended console thing)
would test this too.

It really makes things potentially much simpler. It's not just that it
removes 150 lines, it's 150 lines of  really nasty complex special
case crud.

I'm obviously not going to apply it for 4.9, but it might be "do it
for 4.10 and mark it for stable if nobody notices any issues", because
apart from the code simplification it also gets more cases "right".
Right now the console flushing often means that we end up not merging
continuation lines as much as we could.

           Linus