Messages in this thread | | | Date | Fri, 27 Mar 2015 14:41:44 -0500 (CDT) | From | Christoph Lameter <> | Subject | Re: [for-next][PATCH 1/4] ring-buffer: Replace this_cpu_*() with __this_cpu_*() |
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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.
> 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);
or more compact
unsigned int val = __this_cpu_read(current_context);
__this_cpu_write(current_context, val & (val - 1));
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