On 02/15, Raghavendra K T wrote:
>
> On 02/13/2015 09:02 PM, Oleg Nesterov wrote:
>
>>> @@ -772,7 +773,8 @@ __visible void kvm_lock_spinning(struct arch_spinlock *lock, __ticket_t want)
>>> * check again make sure it didn't become free while
>>> * we weren't looking.
>>> */
>>> - if (ACCESS_ONCE(lock->tickets.head) == want) {
>>> + head = READ_ONCE(lock->tickets.head);
>>> + if (__tickets_equal(head, want)) {
>>> add_stats(TAKEN_SLOW_PICKUP, 1);
>>> goto out;
>>
>> This is off-topic, but with or without this change perhaps it makes sense
>> to add smp_mb__after_atomic(). It is nop on x86, just to make this code
>> more understandable for those (for me ;) who can never remember even the
>> x86 rules.
>
> Hope you meant it for add_stat.
No, no. We need a barrier between set_bit(SLOWPATH) and tickets_equal().
Yes, on x86 set_bit() can't be reordered so smp_mb_*_atomic() is nop, but
it can make the code more understandable.
> yes smp_mb__after_atomic() would be
> harmless barrier() in x86. Did not add this V5 as yoiu though but this
> made me look at slowpath_enter code and added an explicit barrier()
> there :).
Well. it looks even more confusing than a lack of barrier ;)
Oleg.
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