From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
To: Jan Kara <jack@suse.cz>
Cc: Petr Mladek <pmladek@suse.com>,
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Viresh Kumar <viresh.kumar@linaro.org>,
Andrew Morton <akpm@linux-foundation.org>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
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: Sat, 20 Aug 2016 14:24:30 +0900 [thread overview]
Message-ID: <20160820052430.GA695@swordfish> (raw)
In-Reply-To: <20160819190007.GA8275@quack2.suse.cz>
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.
> OTOH there's also the other possible direction for the recursion when we
> are in the scheduler, holding some scheduler locks, decide to WARN which
> enters printk, that ends up calling wake_up_process() which deadlocks
> on scheduler locks... I don't see how to handle this type of recursion
> inside the printk code itself easily and so far the answer was - use
> printk_deferred() in the scheduler and don't use WARN...
the recursion detection is really tricky, yes. it seems (and I haven't
thought of it good enough) to be a bit simpler when we operate in async
printk mode, because we remove this uncontrollable console_unlock().
so we can do something like this:
vprintk_emit(....)
{
local_irq_save();
if (this_cpu_read(in_printk)) {
log_store(BUG: printk recursion!");
goto out;
}
this_cpu_write(in_printk) = 1;
raw_spin_lock(&logbuf_lock);
log_store();
raw_spin_unlock(&logbuf_lock);
if (!in_sched) {
if (console_loglevel != CONSOLE_LOGLEVEL_MOTORMOUTH &&
can_printk_async()) {
printk_kthread_need_flush_console = true;
wake_up_process(printk_kthread);
}
}
this_cpu_write(in_printk) = 0;
out:
local_irq_restore();
}
async printk mode from this point of view is sort of atomic.
we can even set different values of per-CPU `in_printk' on various
stages of printk, which will permit to have better recursion handling.
for example, if we recurse from raw_spin_unlock(&logbuf_lock) then we
must re-init logbuf_lock, because it's 99% corrupted... and so on. but
I haven't really thought of it yet. it obviously doesn't work for sync
printk mode.
> Hum, maybe we could add lockdep annotation to a WARN_ON and BUG_ON macros so
> that it would grab and release console_sem (even if the condition is false).
> That way we'd get lockdep splats for all the possible WARN_ON and BUG_ON
> calls that could deadlock.
hm.
-ss
next prev parent reply other threads:[~2016-08-20 5:25 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 [this message]
2016-08-22 4:15 ` Sergey Senozhatsky
2016-08-23 12:19 ` Petr Mladek
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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