Re: [PATCH 01/10] printk: Avoid deadlock in NMI context

[Andrew Morton]

On Mon, 25 May 2015 14:46:24 +0200 Petr Mladek <pmladek@xxxxxxx> wrote:

> printk() cannot be used in NMI context safely because it uses an internal
> lock and thus could cause a deadlock. This is fine because NMI context
> is very special. The handlers should be short, effective, and do not
> use printk().
> 
> But developers tend to print warnings even from NMI code. They are pretty
> hard to debug when they lockup the machine and nothing appears in the logs.
> 
> This patch prevents from the deadlock on logbuf_lock by using trylock
> rather than spin_lock. If the lock can not be taken, the message is
> ignored and some warning is printed later on.
> 
> We also must not try to get console from NMI context. It needs
> even more locks and there is even higher chance to hung up.
> 
> Unfortunately, we could not print more details about the lost message.
> We could not alloc a buffer in NMI. Therefore we would need some
> lockless mechanism to share a buffer between NMI and normal context.
> But this would make printk() code much more complicated and
> it is not worth it. There has already been an attempt to do so
> and it has been rejected, see https://lkml.org/lkml/2014/6/10/388
> This is also the reason why we use the atomic counter.

hm, I expect it wouldn't be too messy to shove the text into a static
per-cpu buffer.  So we at least get a few hundred bytes of stuff.

Or maybe just save the address of the control string perhaps, so the
message later comes out without any of its %thingies filled in.  That's
racy against rmmod and requires that the control string be in static
storage (which is basically always true).  Of course, it might just be
"%s" ;).  Forget I suggested this.


> +		/* emit KERN_CRIT message */
> +		printed_len += log_store(0, 2, LOG_PREFIX|LOG_NEWLINE, 0,
  +					 NULL, 0, text, text_len);

s/2/LOGLEVEL_CRIT/
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