Re: [PATCH v2] printk: Add console owner and waiter logic to load balance console writes

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 3 Nov 2017 11:19:53 +0100
Jan Kara <jack@suse.cz> wrote:

> Hi,
> 
> On Thu 02-11-17 13:06:05, Steven Rostedt wrote:
> > +			if (spin) {
> > +				/* We spin waiting for the owner to release us */
> > +				spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
> > +				/* Owner will clear console_waiter on hand off */
> > +				while (!READ_ONCE(console_waiter))
> > +					cpu_relax();  
> 
> Hum, what prevents us from rescheduling here? And what if the process
> stored in console_owner is scheduled out? Both seem to be possible with
> CONFIG_PREEMPT kernel? Unless I'm missing something you will need to
> disable preemption in some places...

Yes you are missing something ;-)

> 
> Other than that I like the simplicity of your approach.
> 
> 								Honza
> 
> > +
> > +				spin_release(&console_owner_dep_map, 1, _THIS_IP_);
> > +				printk_safe_exit_irqrestore(flags);

The above line re-enables interrupts. And is done for both the
console_owner and the console_waiter. These are only held with
interrupts disabled. Nothing will preempt it. In fact, if it could,
lockdep would complain (it did in when I screwed it up at first ;-)


-- Steve

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