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percpu: update local_ops.txt to reflect this_cpu operations

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Update the documentation to reflect changes due to the availability of
this_cpu operations.

Signed-off-by: Christoph Lameter <cl@linux.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Christoph Lameter 2014-12-12 16:58:45 -08:00 committed by Linus Torvalds
parent 6c51ec4d18
commit 7d94a82e45
1 changed files with 9 additions and 4 deletions
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13
Documentation/local_ops.txt
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@ -8,6 +8,11 @@ to implement them for any given architecture and shows how they can be used
properly. It also stresses on the precautions that must be taken when reading
those local variables across CPUs when the order of memory writes matters.
Note that local_t based operations are not recommended for general kernel use.
Please use the this_cpu operations instead unless there is really a special purpose.
Most uses of local_t in the kernel have been replaced by this_cpu operations.
this_cpu operations combine the relocation with the local_t like semantics in
a single instruction and yield more compact and faster executing code.
* Purpose of local atomic operations
@ -87,10 +92,10 @@ the per cpu variable. For instance :
local_inc(&get_cpu_var(counters));
put_cpu_var(counters);
If you are already in a preemption-safe context, you can directly use
__get_cpu_var() instead.
If you are already in a preemption-safe context, you can use
this_cpu_ptr() instead.
local_inc(&__get_cpu_var(counters));
local_inc(this_cpu_ptr(&counters));
@ -134,7 +139,7 @@ static void test_each(void *info)
{
/* Increment the counter from a non preemptible context */
printk("Increment on cpu %d\n", smp_processor_id());
local_inc(&__get_cpu_var(counters));
local_inc(this_cpu_ptr(&counters));
/* This is what incrementing the variable would look like within a
* preemptible context (it disables preemption) :