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

[Petr Mladek <pmladek@suse.com>]

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