linux-kernel - Re: [PATCH 1/2] printk: Add new pr_*_deferred

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Date:   Tue, 11 Jan 2022 15:40:43 +0200
From:   Andy Shevchenko <andriy.shevchenko@...el.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Qais Yousef <qais.yousef@....com>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] printk: Add new pr_*_deferred_once() variants

On Mon, Jan 25, 2021 at 12:04:01PM +0106, John Ogness wrote:
> On 2021-01-25, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Sat, Jan 23, 2021 at 11:37:40PM +0000, Qais Yousef wrote:

...

> > We should be very close to printk not needing this anymore, printk
> > people?
> 
> It will disappear once console printing threads are introduced. We
> probably still have a few kernel releases until we see that. First we
> need to finish merging full lockless access, remove the safe buffers,
> and merge the atomic consoles.

Where are we now at this?

I almost easily can reproduce the below at boot time for last several releases

 cblist_init_generic: Setting adjustable number of callback queues.

 =============================
 [ BUG: Invalid wait context ]
 5.16.0-next-20220111-00101-gb3e9f395c6f1 #28 Not tainted
 -----------------------------
 swapper/0/1 is trying to lock:
 ffffffffb3dce758 (&port_lock_key){....}-{3:3}, at: serial8250_console_write+0x3a8/0x480
 other info that might help us debug this:
 context-{5:5}
 3 locks held by swapper/0/1:
  #0: ffffffffb2955e08 (rcu_tasks.cbs_gbl_lock){....}-{2:2}, at: cblist_init_generic.constprop.0+0x14/0x1e0
  #1: ffffffffb2953ec0 (console_lock){+.+.}-{0:0}, at: _printk+0x53/0x6a
  #2: ffffffffb2873ca0 (console_owner){....}-{0:0}, at: console_unlock+0x157/0x580
 stack backtrace:
 CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.16.0-next-20220111-00101-gb3e9f395c6f1 #28
 Hardware name: Intel Corporation CHERRYVIEW D0 PLATFORM/Braswell CRB, BIOS BRAS.X64.B082.R00.1507270557 07/27/2015
 Call Trace:
  <TASK>
  dump_stack_lvl+0x57/0x7d
  __lock_acquire.cold+0xca/0x2e8
  ? stack_trace_save+0x46/0x70
  lock_acquire+0xd2/0x300
  ? serial8250_console_write+0x3a8/0x480
  _raw_spin_lock_irqsave+0x39/0x50
  ? serial8250_console_write+0x3a8/0x480
  serial8250_console_write+0x3a8/0x480
  console_unlock+0x397/0x580
  vprintk_emit+0x99/0x2e0
  _printk+0x53/0x6a
  cblist_init_generic.constprop.0.cold+0x24/0x31
  rcu_init_tasks_generic+0x5/0xd9
  kernel_init_freeable+0x14d/0x294
  ? rest_init+0x250/0x250
  kernel_init+0x11/0x110
  ret_from_fork+0x22/0x30
  </TASK>
 cblist_init_generic: Setting shift to 2 and lim to 1.

-- 
With Best Regards,
Andy Shevchenko