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Re: [RFC][PATCH 0/7] printk: use alt_printk to handle printk() recursive calls

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On Sat 2016-10-01 12:02:51, Sergey Senozhatsky wrote:
> On (09/30/16 13:27), Petr Mladek wrote:
> > > > > 	This patch set extends a lock-less NMI per-cpu buffers idea to
> > > > > handle recursive printk() calls. The basic mechanism is pretty much the
> > > > > same -- at the beginning of a deadlock-prone section we switch to lock-less
> > > > > printk callback, and return back to a default printk implementation at the
> > > > > end; the messages are getting flushed to a logbuf buffer from a safer
> > > > > context.
> > > > 
> > > > I was skeptical but I really like this way now.
> > > >
> > > > The switching of the buffers is a bit hairy in this version but I
> > > > think that we could make it much better.
> > > > 
> > > > Other than that it looks like a big win. It kills a lot of
> > > > printk-related pain points. And it will not be that complicated
> > > > after all.
> > > 
> > > many thanks for looking at this train wreck.
> > > 
> > > so, like I said, it addresses printk()-recursion in *ideally* quite
> > > a minimalistic way -- just several alt_printk_enter/exit calls in
> > > printk.c, without ever touching any other parts of the kernel.
> > > 
> > > gunning down printk deadlocks in general, however, requires much more
> > > effort; or even a completely different approach.
> > > 
> > > a) a lock-less printk() by default
> > >    um, `#define printk alt_printk'. but this will break printk() from irq.
> > >    and the ordering of messages from per-cpu buffers may be far from correct.
> > 
> > Well, the current vprintk_nmi() is lockless. The alternative printk()
> > is going to use the same code, so it will be lockless as well. It
> > means that even this patchset is supposed to avoid all possible
> > deadlocks via printk() calls.
> 
> I meant that printk-recursion and printk-deadlock can be different
> scenarios. deadlocks are harder to handle
> 
>  devkmsg_open()
>   raw_spin_lock_irq(&logbuf_lock)
>    spin_dump()
>     printk()
>      raw_spin_lock_irqsave(&logbuf_lock)
> 
> this one can be handled by alt_printk.
> 
>  devkmsg_open()
>   local_irq_save();
>   alt_printk_enter()
>   raw_spin_lock(&logbuf_lock)
>    spin_dump()
>     printk()
>      vprintk_alt()
> 
> but there are some that can't be handled solely in printk.c

Do you have an example of the still problematic code, please?
vprintk_alt() must be lockless because the same code is used
also in NMI context. If it takes a lock, it is a bug.
Therefore it should not cause a deadlock.

The only problem might be an infinite loop. But the loop should
break once the per-CPU buffer is full. We only need to make sure
that there is no printk() called before the check for the full
buffer. But this reduces the error-prone part of the code to
a minimum. Therefore it should be bearable.

Best Regards,
Petr

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[RFC][PATCH 0/7] printk: use alt_printk to handle printk() recursive calls Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2016-09-27 16:30 +0200
  [RFC][PATCH 5/7] printk: drop vprintk_func function Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2016-09-27 16:30 +0200
  [RFC][PATCH 3/7] printk: introduce per-cpu alt_print seq buffer Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2016-09-27 16:30 +0200
    Re: [RFC][PATCH 3/7] printk: introduce per-cpu alt_print seq buffer Petr Mladek <pmladek@suse.com> - 2016-09-29 14:30 +0200
      Re: [RFC][PATCH 3/7] printk: introduce per-cpu alt_print seq buffer Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-09-30 03:10 +0200
        Re: [RFC][PATCH 3/7] printk: introduce per-cpu alt_print seq buffer Petr Mladek <pmladek@suse.com> - 2016-09-30 13:40 +0200
  [RFC][PATCH 1/7] printk: use vprintk_func in vprintk() Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2016-09-27 16:30 +0200
  [RFC][PATCH 2/7] printk: rename nmi.c and exported api Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2016-09-27 16:30 +0200
  [RFC][PATCH 6/7] printk: use alternative printk buffers Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2016-09-27 16:30 +0200
    Re: [RFC][PATCH 6/7] printk: use alternative printk buffers Petr Mladek <pmladek@suse.com> - 2016-09-29 15:10 +0200
      Re: [RFC][PATCH 6/7] printk: use alternative printk buffers Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-09-30 03:20 +0200
        Re: [RFC][PATCH 6/7] printk: use alternative printk buffers Petr Mladek <pmladek@suse.com> - 2016-09-30 13:20 +0200
          Re: [RFC][PATCH 6/7] printk: use alternative printk buffers Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-10-01 04:50 +0200
            Re: [RFC][PATCH 6/7] printk: use alternative printk buffers Petr Mladek <pmladek@suse.com> - 2016-10-04 14:30 +0200
              Re: [RFC][PATCH 6/7] printk: use alternative printk buffers Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-10-05 03:40 +0200
                Re: [RFC][PATCH 6/7] printk: use alternative printk buffers Petr Mladek <pmladek@suse.com> - 2016-10-05 12:20 +0200
          Re: [RFC][PATCH 6/7] printk: use alternative printk buffers Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2016-10-03 10:00 +0200
          Re: [RFC][PATCH 6/7] printk: use alternative printk buffers Petr Mladek <pmladek@suse.com> - 2016-10-04 17:00 +0200
            Re: [RFC][PATCH 6/7] printk: use alternative printk buffers Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-10-05 03:30 +0200
              Re: [RFC][PATCH 6/7] printk: use alternative printk buffers Petr Mladek <pmladek@suse.com> - 2016-10-05 12:00 +0200
                Re: [RFC][PATCH 6/7] printk: use alternative printk buffers Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-10-06 06:30 +0200
                Re: [RFC][PATCH 6/7] printk: use alternative printk buffers Petr Mladek <pmladek@suse.com> - 2016-10-06 13:40 +0200
                Re: [RFC][PATCH 6/7] printk: use alternative printk buffers Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-10-10 06:20 +0200
                Re: [RFC][PATCH 6/7] printk: use alternative printk buffers Petr Mladek <pmladek@suse.com> - 2016-10-10 13:20 +0200
                Re: [RFC][PATCH 6/7] printk: use alternative printk buffers Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-10-11 09:40 +0200
                Re: [RFC][PATCH 6/7] printk: use alternative printk buffers Petr Mladek <pmladek@suse.com> - 2016-10-11 11:40 +0200
  [RFC][PATCH 4/7] printk: make alt_printk available when config printk set Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2016-09-27 16:30 +0200
  [RFC][PATCH 7/7] printk: new printk() recursion detection Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2016-09-27 16:30 +0200
    Re: [RFC][PATCH 7/7] printk: new printk() recursion detection Petr Mladek <pmladek@suse.com> - 2016-09-29 15:30 +0200
      Re: [RFC][PATCH 7/7] printk: new printk() recursion detection Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-09-30 04:10 +0200
  Re: [RFC][PATCH 0/7] printk: use alt_printk to handle printk()  recursive calls Petr Mladek <pmladek@suse.com> - 2016-09-29 15:30 +0200
    Re: [RFC][PATCH 0/7] printk: use alt_printk to handle printk()  recursive calls Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-09-30 04:50 +0200
      Re: [RFC][PATCH 0/7] printk: use alt_printk to handle printk()  recursive calls Petr Mladek <pmladek@suse.com> - 2016-09-30 13:30 +0200
        Re: [RFC][PATCH 0/7] printk: use alt_printk to handle printk()  recursive calls Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-10-01 05:10 +0200
          Re: [RFC][PATCH 0/7] printk: use alt_printk to handle printk()  recursive calls Petr Mladek <pmladek@suse.com> - 2016-10-04 13:40 +0200

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