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SubjectRe: [PATCH 05/10] nohz: New tick dependency mask
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On 07/24/2015 01:16 PM, Frederic Weisbecker wrote:
> On Fri, Jul 24, 2015 at 12:55:35PM -0400, Chris Metcalf wrote:
>> On 07/23/2015 12:42 PM, Frederic Weisbecker wrote:
>>> +unsigned long __tick_nohz_set_tick_dependency(enum tick_dependency_bit bit,
>>> + unsigned long *dep)
>>> +{
>>> + unsigned long prev;
>>> + unsigned long old = *dep;
>>> + unsigned long mask = BIT_MASK(bit);
>>> +
>>> + while ((prev = cmpxchg(dep, old, old | mask)) != old) {
>>> + old = prev;
>>> + cpu_relax();
>>> + }
>>> +
>>> + return prev;
>>> +}
>> Why not use set_bit() here? It is suitably atomic.
> Because I don't want to send an IPI if the CPU already had bits set in
> the dependency.
>
> Ideally I need something like test_and_set_bit() but which returns the
> whole previous value and not just the previous value of the bit.

Ah, of course. Peter, maybe we need atomic_or_return() as part
of your new atomic_or/_and/_xor series? Certainly on tilegx, and
likely other architectures, we can do better than Frederic's
cmpxchg() loop.

--
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com



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