Re: [RFC][PATCH 3/7] printk: introduce per-cpu alt_print seq buffer

From: Sergey Senozhatsky
Date: Thu Sep 29 2016 - 21:05:43 EST

On (09/29/16 14:26), Petr Mladek wrote:
> > printk()
> > local_irq_save()
> > alt_printk_enter()
> We need to make sure that exit() is called on the same CPU.
> Therefore we need to disable preemption as well.

local_irq_save() does this for us, we can't get sched tick or
re-sched IPI, and even more - we eliminate race conditions on
this CPU. only one path can touch alt_printk related stuff,
NMI works with its own buffer.

> What do you think about my approach with the printk_context per-CPU
> value from the WARN_DEFERRED() patchset? The main idea is that
> the entry()/exit() functions manipulate preempt_count-like per-CPU
> variable. The printk() function selects the safe implementation
> according to the current state.

I'll take a look.

hm, what I was thinking of... you are right, this all smells a bit
bad. I'll revisit it.