[PATCH] [RFC] nptl: use compare and exchange for lll_cond_lock

[Adhemerval Zanella]

On 24-09-2014 10:46, Bernie Ogden wrote:
> IIUC the two options are indeed equivalent - but wouldn't it be more
> efficient to synchronize on the non-comparing version?

Well, that is exactly the question I posed ;)

>
> On 23 September 2014 18:45, Adhemerval Zanella
> <azanella@linux.vnet.ibm.com> wrote:
>> While checking the generated code and macros used in generic lowlevellock.h,
>> I noted powerpc and other arch uses uses a compare and swap instead of a plain
>> exchange value on lll_cond_lock.
>>
>> I am not really sure which behavior would be desirable, since as far I could
>> they will have both the same side effects (since lll_cond_lock, different
>> from lll_lock, does not hold value of '1').
>>
>> So I am proposing this patch to sync default implementation for what mostly
>> architectures (ia64, ppc, s390, sparc, x86, hppa) uses for lll_cond_lock.  I see
>> that only microblaze and sh (I am not sure about this one, I not well versed in
>> its assembly and I'm being guided by its comment) used the atomic_exchange_acq
>> instead.
>>
>> Checked on powerpc32 and powercp64 with my previous lowlevellock.h removal
>> patch.
>>
>> --
>>
>>         * sysdeps/nptl/lowlevellock.h (__lll_cond_lock): Use
>>         atomic_compare_and_exchange_val_acq instead of atomic_exchange_acq.
>>
>> ---
>>
>> diff --git a/sysdeps/nptl/lowlevellock.h b/sysdeps/nptl/lowlevellock.h
>> index 28f4ba3..ba22734 100644
>> --- a/sysdeps/nptl/lowlevellock.h
>> +++ b/sysdeps/nptl/lowlevellock.h
>> @@ -73,7 +73,8 @@ extern int __lll_robust_lock_wait (int *futex, int private) attribute_hidden;
>>    ((void)                                                               \
>>     ({                                                                   \
>>       int *__futex = (futex);                                            \
>> -     if (__glibc_unlikely (atomic_exchange_acq (__futex, 2) != 0))      \
>> +     if (__glibc_unlikely (                                            \
>> +        atomic_compare_and_exchange_val_acq (__futex, 2, 0) != 0))     \
>>         __lll_lock_wait (__futex, private);                              \
>>     }))
>>  #define lll_cond_lock(futex, private) __lll_cond_lock (&(futex), private)
>>