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

[Sergey Senozhatsky]

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

	-ss