Re: [PATCH] x86 spinlock: Fix memory corruption on completing completions

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

On 02/10/2015 06:56 PM, Oleg Nesterov wrote:
> On 02/10, Raghavendra K T wrote:
>>
>> On 02/10/2015 06:23 AM, Linus Torvalds wrote:
>>
>>>           add_smp(&lock->tickets.head, TICKET_LOCK_INC);
>>>           if (READ_ONCE(lock->tickets.tail) & TICKET_SLOWPATH_FLAG) ..
>>>
>>> into something like
>>>
>>>           val = xadd((&lock->ticket.head_tail, TICKET_LOCK_INC << TICKET_SHIFT);
>>>           if (unlikely(val & TICKET_SLOWPATH_FLAG)) ...
>>>
>>> would be the right thing to do. Somebody should just check that I got
>>> that shift right, and that the tail is in the high bytes (head really
>>> needs to be high to work, if it's in the low byte(s) the xadd would
>>> overflow from head into tail which would be wrong).
>>
>> Unfortunately xadd could result in head overflow as tail is high.
>>
>> The other option was repeated cmpxchg which is bad I believe.
>> Any suggestions?
>
> Stupid question... what if we simply move SLOWPATH from .tail to .head?
> In this case arch_spin_unlock() could do xadd(tickets.head) and check
> the result

It is a good idea. Trying this now.

> In this case __ticket_check_and_clear_slowpath() really needs to cmpxchg
> the whole .head_tail. Plus obviously more boring changes. This needs a
> separate patch even _if_ this can work.

Correct, but apart from this, before doing xadd in unlock,
we would have to make sure lsb bit is cleared so that we can live with 1 
bit overflow to tail which is unused. now either or both of head,tail
lsb bit may be set after unlock.