Re: [PATCHv3 3/4] xen: use ticket locks for spin locks

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 23/04/15 14:43, David Vrabel wrote:
> On 23/04/15 13:03, Tim Deegan wrote:
>> Hi,
>>
>> At 11:11 +0100 on 21 Apr (1429614687), David Vrabel wrote:
>>>  void _spin_lock(spinlock_t *lock)
>>>  {
>>> +    spinlock_tickets_t tickets = { .tail = 1, };
>>>      LOCK_PROFILE_VAR;
>>>  
>>>      check_lock(&lock->debug);
>>> -    while ( unlikely(!_raw_spin_trylock(&lock->raw)) )
>>> +    tickets.head_tail = xadd(&lock->tickets.head_tail, tickets.head_tail);
>>> +    while ( tickets.tail != observe_head(&lock->tickets) )
>>>      {
>>>          LOCK_PROFILE_BLOCK;
>>> -        while ( likely(_raw_spin_is_locked(&lock->raw)) )
>>> -            cpu_relax();
>>> +        cpu_relax();
>>>      }
>>>      LOCK_PROFILE_GOT;
>>>      preempt_disable();
>> I think you need an smp_mb() here to make sure that locked accesses
>> don't get hoisted past the wait-for-my-ticket loop by an out-of-order
>> (ARM) cpu.
> Ok, but smp_mb() turns into an mfence on x86.  Is this a
> problem/sub-optimal?

That can probably change back to a plain compiler barrier as Xen only
supports 64bit and doesn't need PPRO ordering errata workarounds.

~Andrew