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Date: Mon, 29 Jun 2015 11:32:19 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Tejun Heo <tj@kernel.org>
Cc: Nicholas Mc Guire <der.herr@hofr.at>, oleg@redhat.com,
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Subject: Re: [RFC][PATCH 05/13] percpu-rwsem: Optimize readers and reduce
 global impact
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On Thu, Jun 25, 2015 at 03:17:01PM -0400, Tejun Heo wrote:

> Hmmm... the only worry I have about this is people using it on u64 on
> 32bit machines.  CPU local ops can do split updates on lower and upper
> halves and the remotely-read value will be surprising.  We have the
> same issues w/ regular per_cpu accesses to but the summing function /
> macro is better at giving the false sense of security.  Prolly
> limiting it upto ulong size is a good idea?

Agreed, luckily we already have the infrastructure for this, something
like so?

--- a/include/linux/percpu-defs.h
+++ b/include/linux/percpu-defs.h
@@ -287,6 +287,16 @@ do {									\
 	preempt_enable();						\
 } while (0)
 
+#define per_cpu_sum(var)						\
+({									\
+	typeof(var) __sum = 0;						\
+	int cpu;							\
+	compiletime_assert_atomic_type(__sum);				\
+	for_each_possible_cpu(cpu)					\
+		__sum += per_cpu(var, cpu);				\
+	__sum;								\
+})
+
 /*
  * Branching function to split up a function into a set of functions that
  * are called for different scalar sizes of the objects handled.