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Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async

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On (03/16/16 14:39), Byungchul Park wrote:
> On Tue, Mar 15, 2016 at 11:07:38PM +0900, Sergey Senozhatsky wrote:
> > 
> > something like this (*minimally tested so far*).
> > 
> > -- move wake_up() and friends under the logbuf section; so we can detect
>                                           ^^^^^^^^^^^^^^
> 					  section protected by logbuf_lock?
> 
> >    printk() recursion from wake_up()
> 
> Excuse me, but I fear that it can cause an unnecessary deadlock.

where? shouldn't it be

vprintk_emit()
  local_irq_save(flags);
  this_cpu = smp_processor_id();

  raw_spin_lock(&logbuf_lock);
  static logbuf_cpu = this_cpu;

     wake_up_process()
      spin_lock()
       spin_dump()
        vprintk_emit()				<< recursion
         local_irq_save(flags);
         this_cpu = smp_processor_id();

         if (unlikely(logbuf_cpu == this_cpu)) {
            recursion_bug = true;
            return;   				<< break recursion
         }

?

> Furthermore it cannot be handled if it is caused by logbuf_lock.

sure, I'm not even trying to address a logbuf spin_dump recursion problem here.
I'm only trying to minimize the impact of newly introduced spin locks.

I don't have a very clear vision (at the moment) of how to fix printk recursion
caused by logbuf lock of console_sem corruptions, etc. Should spin_dump
be aware of the locks that can be taken by printk()? Hm, I can't even count all
the locks that possibly can be taken by printk->console_drivers and most likely
don't even see all of the cases where printk can recurse. Disable lock debug
in vprintk_emit() the same way lockdep is desabled? Hm...

Ingo's POV is that printk must be reworked and become smarter in this aspect.

> I mean that it would be better to keep the wake_up and friend out of the
> critical section by logbuf_lock.

in this case wake_up_process() can recurse us forever.

	-ss

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[RFC][PATCH v4 0/2] printk: Make printk() completely async Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2016-03-14 15:20 +0100
  [RFC][PATCH v4 1/2] printk: Make printk() completely async Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2016-03-14 15:20 +0100
    Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async Jan Kara <jack@suse.cz> - 2016-03-15 11:10 +0100
      Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2016-03-15 15:20 +0100
        Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async Byungchul Park <byungchul.park@lge.com> - 2016-03-16 06:50 +0100
          Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-03-16 08:00 +0100
            Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async Byungchul Park <byungchul.park@lge.com> - 2016-03-16 08:40 +0100
              Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-03-16 09:00 +0100
                Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async Byungchul Park <byungchul.park@lge.com> - 2016-03-16 11:40 +0100
                Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-03-17 01:40 +0100
          Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async Byungchul Park <byungchul.park@lge.com> - 2016-03-16 08:10 +0100
            Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-03-16 08:10 +0100
    Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async Petr Mladek <pmladek@suse.com> - 2016-03-15 17:00 +0100
      Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-03-16 03:10 +0100
        Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async Byungchul Park <byungchul.park@lge.com> - 2016-03-16 03:20 +0100
          Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-03-16 03:40 +0100
  [RFC][PATCH v4 2/2] printk: Skip messages on oops Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2016-03-14 15:20 +0100
    Re: [RFC][PATCH v4 2/2] printk: Skip messages on oops Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-03-17 12:00 +0100

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