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: Thu, 25 Aug 2016 23:10:00 +0200 [thread overview]
Message-ID: <20160825210959.GA2273@dhcp128.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?
> > > >
> since we are basically interested in wake_up_process() only from
> printk() POV. not sure how acceptable 2 * preempt_count and 2 * per-CPU
> writes for every try_to_wake_up().
>
>
> 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();
>
> /*
> @@ -1804,6 +1805,7 @@ asmlinkage int vprintk_emit(int facility, int level,
> */
> if (!oops_in_progress && !lockdep_recursing(current)) {
> recursion_bug = true;
> + printk_nmi_exit();
> local_irq_restore(flags);
> return 0;
> }
> @@ -1920,6 +1922,7 @@ asmlinkage int vprintk_emit(int facility, int level,
> logbuf_cpu = UINT_MAX;
> raw_spin_unlock(&logbuf_lock);
> lockdep_on();
> + printk_nmi_exit();
> local_irq_restore(flags);
>
> /* If called from the scheduler, we can not call up(). */
I was so taken by the idea of temporary forcing a lockless and
"trivial" printk implementation that I missed one thing.
Your patch use the alternative printk() variant around logbuf_lock.
But this is not the problem with wake_up_process(). printk_deferred()
takes logbuf_lock without problems.
Our problem is with calling wake_up_process() recursively. The
deadlock is in the scheduler locks.
But the patch still inspired me. What about blocking the problematic
wake_up_process() call by a per-cpu variable. I mean something like
this completely untested code:
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index ca9733b802ce..93915eb1fd0d 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1708,6 +1708,8 @@ static size_t cont_print_text(char *text, size_t size)
return textlen;
}
+DEFINE_PER_CPU(bool, printk_wakeup);
+
asmlinkage int vprintk_emit(int facility, int level,
const char *dict, size_t dictlen,
const char *fmt, va_list args)
@@ -1902,8 +1904,17 @@ asmlinkage int vprintk_emit(int facility, int level,
lockdep_off();
if (printk_kthread && !in_panic) {
+ bool __percpu *printk_wakeup_ptr;
+
/* Offload printing to a schedulable context. */
- wake_up_process(printk_kthread);
+ local_irq_save(flags);
+ printk_wake_up_ptr = this_cpu_ptr(&printk_wake_up);
+ if (!*printk_wakeup_ptr) {
+ *printk_wake_up_ptr = true;
+ wake_up_process(printk_kthread);
+ *printk_wake_up_ptr = false;
+ }
+ local_irq_restore(flags);
goto out_lockdep;
} else {
/*
We might eventually hide this into a wake_up_process_safe() or so.
Also we might need to use it also in console_unlock() to avoid similar
recursion there as well.
Best Regards,
Petr
next prev parent reply other threads:[~2016-08-25 21:23 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
2016-08-24 1:33 ` Sergey Senozhatsky
2016-08-25 21:10 ` Petr Mladek [this message]
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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