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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