linux-kernel - Re: [RFC][PATCH 3/7] printk: introduce per-cpu alt

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Date:   Fri, 30 Sep 2016 13:35:08 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Calvin Owens <calvinowens@...com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 3/7] printk: introduce per-cpu alt_print seq buffer

On Fri 2016-09-30 10:05:28, Sergey Senozhatsky wrote:
> 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.
> 
> I'll revisit it.

Just a side note. If you make it less generic then please use some more
meaning-full name for the alternative printk stuff. The following
comes to my mind:

	     printk_safe_enter();
	     printk_safe_exit();
	     vprintk_safe();

IMHO, the "_safe" suffix often means a variant that prevents a
possible deadlock.

I wonder where this patches would end but it looks promising.

Best Regards,
Petr