linux-kernel - Re: [PATCH printk v1 11/13] printk: reimplement console

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Date:   Fri, 11 Mar 2022 17:17:48 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH printk v1 11/13] printk: reimplement console_lock for
 proper kthread support

On Fri 2022-03-11 14:34:40, John Ogness wrote:
> On 2022-03-11, Petr Mladek <pmladek@...e.com> wrote:
> > But it does not help when the lock was taken
> > via console_trylock() from printk(). It might mean that
> > the forward progress might not be guaranteed in the direct mode
> > (early boot, panic, ...).
> 
> How is the console_trylock() case different from current mainline now?
> As I mentioned above, the kthreads can block console_trylock(), but so
> can a console_lock() currently in mainline.

CPU0			CPU1			CPU2

printk()
  // direct mode allowed
  console_trylock()
  console_unlock()
    console_flush_all()

			printk_direct_enter()

      allows_direct_printing() -> false;
      break;

      __console_unlock()
        wakeup_klogd()

						// woken printk_khread
						console_thread_printing_enter()

			printk_direct_exit()

      console_trylock()
        atomic_tryblock()
	  //fails because thread active

   return;

			printk_direct_enter()

						console_thread_printing_exit()

						// sleep because
						atomic_read(&printk_direct) is not
						zero

Result: nobody prints

Note: The thread will actually not sleep because printk_should_wake()
      does not check atomic_read(&printk_direct).

      But it is a bug. Active thread should check it and allow
      entering direct mode.


Note2: Even when one printk thread flushes the messages. There might
       be other thread that will never get scheduled a nobody would
       printk the messages on the related console.


This is the race that I see with console_trylock(). IMHO, if we solve
this race then we do not need console_trylock_sched().

Well, I might be wrong. It is Friday evening. I still do not have
the entire picture. I probably should have waited for Monday.

I am sure that I misunderstood several things, including wait_event()
API or CON_DIRECT flag. But the above race looks real. And it somehow
supports my feeling about that there are some races that can't be
solved by console_trylock_sched().

Best Regards,
Petr