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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