Re: [PATCH 3/6] x86/ticketlock: Use C for __ticket_spin_unlock

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 01/24/2011 05:13 PM, Nick Piggin wrote:
> On Tue, Jan 25, 2011 at 10:41 AM, Jeremy Fitzhardinge <jeremy@goop.org> wrote:
>> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
>>
>> If we don't need to use a locked inc for unlock, then implement it in C.
>>
>> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
>> ---
>>  arch/x86/include/asm/spinlock.h |   32 +++++++++++++++++---------------
>>  1 files changed, 17 insertions(+), 15 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
>> index f48a6e3..0170ba9 100644
>> --- a/arch/x86/include/asm/spinlock.h
>> +++ b/arch/x86/include/asm/spinlock.h
>> @@ -33,9 +33,21 @@
>>  * On PPro SMP or if we are using OOSTORE, we use a locked operation to unlock
>>  * (PPro errata 66, 92)
>>  */
>> -# define UNLOCK_LOCK_PREFIX LOCK_PREFIX
>> +static __always_inline void __ticket_unlock_release(struct arch_spinlock *lock)
>> +{
>> +       if (sizeof(lock->tickets.head) == sizeof(u8))
>> +               asm (LOCK_PREFIX "incb %0"
>> +                    : "+m" (lock->tickets.head) : : "memory");
>> +       else
>> +               asm (LOCK_PREFIX "incw %0"
>> +                    : "+m" (lock->tickets.head) : : "memory");
>> +
>> +}
>>  #else
>> -# define UNLOCK_LOCK_PREFIX
>> +static __always_inline void __ticket_unlock_release(struct arch_spinlock *lock)
>> +{
>> +       lock->tickets.head++;
>> +}
>>  #endif
>>
>>  /*
>> @@ -93,14 +105,6 @@ static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
>>
>>        return tmp;
>>  }
>> -
>> -static __always_inline void __ticket_spin_unlock(arch_spinlock_t *lock)
>> -{
>> -       asm volatile(UNLOCK_LOCK_PREFIX "incb %0"
>> -                    : "+m" (lock->slock)
>> -                    :
>> -                    : "memory", "cc");
>> -}
>>  #else
>>  static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
>>  {
>> @@ -144,15 +148,13 @@ static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
>>
>>        return tmp;
>>  }
>> +#endif
>>
>>  static __always_inline void __ticket_spin_unlock(arch_spinlock_t *lock)
>>  {
>> -       asm volatile(UNLOCK_LOCK_PREFIX "incw %0"
>> -                    : "+m" (lock->slock)
>> -                    :
>> -                    : "memory", "cc");
>> +       __ticket_unlock_release(lock);
>> +       barrier();              /* prevent reordering into locked region */
>>  }
>> -#endif
> The barrier is wrong.

In what way?  Do you think it should be on the other side?

> What makes me a tiny bit uneasy is that gcc is allowed to implement
> this any way it wishes. OK there may be a NULL intersection of possible
> valid assembly which is a buggy unlock... but relying on gcc to implement
> lock primitives is scary. Does this really help in a way that can't be done
> with the assembly versions?

We rely on C/gcc for plenty of other subtle ordering things.  Spinlocks
aren't particularly special in this regard.

    J