On 05/26/2014 12:23 PM, Tejun Heo wrote:
> On Thu, May 22, 2014 at 10:21:25PM +0800, Lai Jiangshan wrote:
>> On Thu, May 22, 2014 at 9:47 PM, Tejun Heo <t...@kernel.org> wrote:
>> This is not busy wait, the retry and numa_pwq_tbl() guarantee that
>> the retry will get a new pwq (even without cpu_relax()) as the comments says,
>
> Yes, *eventually*. It's not guaranteed to succeed on the immediate
> next try. This is a busy wait.
changing pwq:
install pwq
lock(pool->lock)
put_pwq();
unlock(pool->lock)
__queue_work():
lock(pool->lock)
test ref and find it zero;
see the installation here;
it is guaranteed to get the installed pwq on the immediate next try.
unlock()
retry.
>
>> and the refcnt of this new pwq is very very likely non-zero and
>> cpu_relax() can't
>> increase the probability of non-zero-refcnt. cpu_relax() is useless here.
>>
>> It is different from spin_lock() or some other spin code.
>>
>> it is similar to the loop of __task_rq_lock() which also guarantees progress.
>
> No, it's not. __task_rq_lock() *already* sees the updated value to
> use for the next time. Here, we see the old one dead and the new one
> is guaranteed to show up pretty soon but we're still busy waiting for
> it.
>
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