Re: [Xen-devel] [PATCH V4] x86 spinlock: Fix memory corruption on completing completions

[Oleg Nesterov <oleg@xxxxxxxxxx>]

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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