Re: [GIT PULL] printk for 4.13

[Sergey Senozhatsky]

On (07/06/17 09:44), Sergey Senozhatsky wrote:
> > No, this is entirely buggered.
> > 
> > You can't just do
> > 
> >    raw_spin_is_locked(&logbuf_lock)
> > 
> > to test whether you can get the logbuf_lock and then call vprintk_default().
> > 
> > It's not just about deadlock avoidance, the code will call things like
> > down_trylock() on the console semaphore, and that operation is
> > fundamentally not NMI-safe. The semaphore count is literally protected
> > by a irq-safe (BUT NOT NMI-SAFE!) semaphore spinlock.
> >
> > So now you can instead deadlock just on the internal console semaphore
> > spinlock (ie somebody is doing "console_lock()", NMI comes in, *BOOM*.
> 
> you are right. but
> 

just in case if my previous email was too hard to follow, may be the
link below will explain it in a less broken language. (I'm sorry, I'm
on medication now and didn't have enough coffee yet.)

https://marc.info/?l=linux-kernel&m=149265908228203&w=2

	-ss