Re: [for-next][PATCH 1/4] ring-buffer: Replace this_cpu_*() with __this_cpu_*()

[Steven Rostedt]

On Fri, 27 Mar 2015 14:41:44 -0500 (CDT)
Christoph Lameter <cl@xxxxxxxxx> wrote:

> On Wed, 25 Mar 2015, Steven Rostedt wrote:
> 
> > It has come to my attention that this_cpu_read/write are horrible on
> > architectures other than x86. Worse yet, they actually disable
> > preemption or interrupts! This caused some unexpected tracing results
> > on ARM.
> 
> This isnt something new and I thought the comment was dropped from the
> patch? This is a plain error in using this_cpu_* where __this_cpu_* would
> have been sufficient. Code was uselessly disabling preemption twice.
> 

Where in the patch do you see the comment? Or were you talking about
the change log? The original patch did have a comment, an it was
dropped, that's what I thought you were talking about.

> > Which is unacceptable for locations that know they are within preempt
> > disabled or interrupt disabled locations.
> 
> Well yes. Thats why the __this_cpu ops are there to avoid this
> overhead.
> 
> > I also changed the recursive_unlock() to use two local variables instead
> > of accessing the per_cpu variable twice.
> 
> Ok gotta look at that.
> 
> >  static __always_inline void trace_recursive_unlock(void)
> >  {
> > -	unsigned int val = this_cpu_read(current_context);
> > +	unsigned int val = __this_cpu_read(current_context);
> >
> > -	val--;
> > -	val &= this_cpu_read(current_context);
> > -	this_cpu_write(current_context, val);
> > +	val &= val & (val - 1);
> > +	__this_cpu_write(current_context, val);
> >  }
> 
> Ummm... This is does not look like an equivalent thing. Should this not
> be:
> 
> 	unsigned int val = __this_cpu_read(current_context);
> 	unsigned int newval = val - 1;
> 
> 	newval &= val;
> 	__this_cpu_write(current_context, newval);

Actually, it is equivalent, but I do see a issue with my patch.

	val &= val & (val - 1);

is the same as the more reasonable:

	val &= val - 1;

I think I meant to replace &= with = :-/

> 
> or more compact
> 
> 	unsigned int val = __this_cpu_read(current_context);
> 
> 	__this_cpu_write(current_context, val & (val - 1));

Maybe I'll just use your compact version.

Thanks,

-- Steve
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