Re: [RFC][PATCH 6/7] printk: use alternative printk buffers

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

On (09/29/16 15:00), Petr Mladek wrote:
[..]
> > @@ -1791,7 +1791,7 @@ asmlinkage int vprintk_emit(int facility, int level,
> >  		zap_locks();
> >  	}
> >  
> > -	lockdep_off();
> > +	alt_printk_enter();
> 
> IMHO, we could not longer enter vprintk_emit() recursively. The same
> section that was guarded by logbuf_cpu is guarded by
> alt_printk_enter()/exit() now.

you might be very right here. I'll take a look.

> IMHO, we could remove all the logic around the recursion. Then we
> could even disable/enable irqs inside alt_printk_enter()/exit().

I was thinking of doing something like this; but that would require
storing 'unsigned long' flags in per-cpu data

	alt_enter()
	{
		unsinged long flags;

		local_irq_save(flags);
		ctx = this_cpu_ptr();
		ctx->flags = flags;
		...
	}

	alt_exit()
	{
		ctx = this_cpu_ptr();
		...
		local_irq_restore(ctx->flags);
	}


and the decision was to keep `unsigned long flags' on stack in the
alt_enter/exit caller. besides in most of the cases we already have
it (in vprintk_emit() and console_unlock()).

but I can certainly hide these details in alt_enter/exit.


> And to correct myself from the previous mail. It is enough to disable
> IRQs. It is enough to make sure that we will not preempt and will
> stay on the same CPU.

ah, no prob.

> > @@ -2479,7 +2490,9 @@ void console_unlock(void)
> >  	 */
> >  	raw_spin_lock(&logbuf_lock);
> >  	retry = console_seq != log_next_seq;
> > -	raw_spin_unlock_irqrestore(&logbuf_lock, flags);
> > +	raw_spin_unlock(&logbuf_lock);
> > +	alt_printk_exit();
> > +	local_irq_restore(flags);
> 
> We should mention that this patch makes an obsolete artefact from
> printk_deferred(). It opens the door for another big cleanup and
> relief.

do you mean that, once alt_printk is done properly, we can drop
printk_deferred()? I was thinking of it, but decided not to
mention/touch it in this patch set.

	-ss