From: Petr Mladek <pmladek@suse.com>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Jan Kara <jack@suse.cz>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Viresh Kumar <viresh.kumar@linaro.org>,
Andrew Morton <akpm@linux-foundation.org>,
Jan Kara <jack@suse.com>, Tejun Heo <tj@kernel.org>,
Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Byungchul Park <byungchul.park@lge.com>,
vlevenetz@mm-sol.com,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH v10 1/2] printk: Make printk() completely async
Date: Tue, 23 Aug 2016 14:19:03 +0200 [thread overview]
Message-ID: <20160823121903.GD4866@pathway.suse.cz> (raw)
In-Reply-To: <20160822041520.GA511@swordfish>
On Mon 2016-08-22 13:15:20, Sergey Senozhatsky wrote:
> Hello,
>
> On (08/20/16 14:24), Sergey Senozhatsky wrote:
> > On (08/19/16 21:00), Jan Kara wrote:
> > > > > depending on .config BUG() may never return back -- passing control
> > > > > to do_exit(), so printk_deferred_exit() won't be executed. thus we
> > > > > probably need to have a per-cpu variable that would indicate that
> > > > > we are in deferred_bug. hm... but do we really need deferred BUG()
> > > > > in the first place?
> > > >
> > > > Good question. I am not aware of any BUG_ON() that would be called from
> > > > wake_up_process() but it is hard to check everything.
> > > >
> > > > A conservative approach would be to force synchronous printk from
> > > > BUG_ON().
> > >
> > > Just a quick thought: Cannot we just do printk_deferred_enter() when we are
> > > about to call into the scheduler from printk code and printk_deferred_exit()
> > > when leaving it? That would look like the least error-prone way how
> > > handling this kind of recursion...
> >
> > interesting idea.
> > printk_deferred_enter() increments preempt count, so there may be additional
> > obstacles and, as a result, ad-hocs, that scheduler people will sincerely hate.
> > need to think more.
>
> the other thing I just thought of is doing something as follows
> !!!not tested, will not compile, just an idea!!!
>
> ---
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 6e260a0..bb8d719 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1789,6 +1789,7 @@ asmlinkage int vprintk_emit(int facility, int level,
> printk_delay();
>
> local_irq_save(flags);
> + printk_nmi_enter();
> this_cpu = smp_processor_id();
Huh, this looks very interesting but I am afraid that it will not fly.
The problem is that vprintk_nmi() is safe only when it is used
exclusively in NMI.
The following could happen with your code:
/**** normar context ****/
vprintk_emit()
printk_nmi_enter()
...
wake_up_process()
WARN()
printk()
vprintk_nmi()
vsnprintf(..., "0123456789")
/* real NMI comes after writing "01234" */
/**** NMI context ****/
vprintk_nmi();
vsnprintf(..., "abcdefghijklmno");
/**** normal context ****/
/* we finish writing "56789" into the buffer */
=> part of the message from NMI gets broken "abcde56789klmno".
The lockless handling of the NMI per-CPU buffer already is not
trivial. I would be afraid to add more hacks to make
it writable in all contexts.
I am sorry about the bad news. This was so promising on the first
look.
Best Regards,
Petr
next prev parent reply other threads:[~2016-08-23 12:19 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-04 16:57 [PATCH v10 0/2] printk: Make printk() completely async Sergey Senozhatsky
2016-04-04 16:57 ` [PATCH v10 1/2] " Sergey Senozhatsky
2016-04-04 22:51 ` Andrew Morton
2016-04-05 5:17 ` Sergey Senozhatsky
2016-04-05 7:39 ` Sergey Senozhatsky
2016-04-06 0:19 ` Sergey Senozhatsky
2016-04-06 8:27 ` Jan Kara
2016-04-07 9:48 ` Sergey Senozhatsky
2016-04-07 12:08 ` Sergey Senozhatsky
2016-04-07 13:15 ` Jan Kara
2016-08-10 21:17 ` Viresh Kumar
2016-08-12 9:44 ` Petr Mladek
2016-08-15 14:26 ` Vladislav Levenetz
2016-08-16 9:04 ` Petr Mladek
2016-08-18 2:27 ` Sergey Senozhatsky
2016-08-18 9:33 ` Petr Mladek
2016-08-18 9:51 ` Sergey Senozhatsky
2016-08-18 10:56 ` Petr Mladek
2016-08-19 6:32 ` Sergey Senozhatsky
2016-08-19 9:54 ` Petr Mladek
2016-08-19 19:00 ` Jan Kara
2016-08-20 5:24 ` Sergey Senozhatsky
2016-08-22 4:15 ` Sergey Senozhatsky
2016-08-23 12:19 ` Petr Mladek [this message]
2016-08-24 1:33 ` Sergey Senozhatsky
2016-08-25 21:10 ` Petr Mladek
2016-08-26 1:56 ` Sergey Senozhatsky
2016-08-26 8:20 ` Sergey Senozhatsky
2016-08-30 9:29 ` Petr Mladek
2016-08-31 2:31 ` Sergey Senozhatsky
2016-08-31 9:38 ` Petr Mladek
2016-08-31 12:52 ` Sergey Senozhatsky
2016-09-01 8:58 ` Petr Mladek
2016-09-02 7:58 ` Sergey Senozhatsky
2016-09-02 15:15 ` Petr Mladek
2016-09-06 7:16 ` Sergey Senozhatsky
2016-08-23 13:03 ` Petr Mladek
2016-08-23 13:48 ` Petr Mladek
2016-04-04 16:57 ` [PATCH v10 2/2] printk: Make wake_up_klogd_work_func() async Sergey Senozhatsky
2016-08-23 3:32 [PATCH v10 1/2] printk: Make printk() completely async Andreas Mohr
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