Re: [RFC PATCH 5/6] implement per-cpu&per-domain state machine call_srcu()

[Gilad Ben-Yossef]

On Tue, Mar 6, 2012 at 4:44 PM, Lai Jiangshan <eag0628@xxxxxxxxx> wrote:
> On Tue, Mar 6, 2012 at 7:52 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>> On Tue, 2012-03-06 at 17:57 +0800, Lai Jiangshan wrote:
>>> +void srcu_barrier(struct srcu_struct *sp)
>>> +{
>>> +       struct srcu_sync sync;
>>> +       struct srcu_head *head = &sync.head;
>>> +       unsigned long chck_seq; /* snap */
>>> +
>>> +       int idle_loop = 0;
>>> +       int cpu;
>>> +       struct srcu_cpu_struct *scp;
>>> +
>>> +       spin_lock_irq(&sp->gp_lock);
>>> +       chck_seq = sp->chck_seq;
>>> +       for_each_possible_cpu(cpu) {
>>
>> ARGH!! this is really not ok.. so we spend all this time killing
>> srcu_sync_expidited and co because they prod at all cpus for no good
>> reason, and what do you do?
>
> it is srcu_barrier(), it have to wait all callbacks complete for all
> cpus since it is per-cpu
> implementation.

I would say it only needs to wait for callbacks to complete for all
CPUs that has a callback pending.

Unless I misunderstood something, that is what your code does already
- it does not wait for completion,
or schedules a work on a CPU that does not has a callback pending, right?

>
>>
>> Also, what happens if your cpu isn't actually online?
>
> The workqueue handles it, not here, if a cpu state machine has callbacks, the
> state machine is started, if it has no callback,  srcu_barrier() does
> nothing for
> this cpu

I understand the point is that offline cpus wont have callbacks, so
nothing would be
done for them, but still, is that a reason to even check? why not use
for_each_online_cpu

I think that if a cpu that was offline went online after your check
and managed to get an
SRCU callback pending it is by definition not a callback srcu_barrier
needs to wait for
since it went pending at a later time then srcu_barrier was called. Or
have I missed something?

Thanks,
Gilad

-- 
Gilad Ben-Yossef
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gilad@xxxxxxxxxxxxx
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