Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write_unlock()

[Daniel J Blueman <daniel@numascale.com>]

On 01/15/2014 07:44 AM, Paul E. McKenney wrote:
> On Tue, Jan 14, 2014 at 10:01:04AM -0800, Richard Henderson wrote:
>> On 01/14/2014 09:08 AM, Matt Turner wrote:
>>> On Tue, Jan 14, 2014 at 3:03 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>>>> On Tue, Jan 14, 2014 at 10:28:23AM +0800, Daniel J Blueman wrote:
>>>>>> Peter,
>>>>>>
>>>>>> I found out that the build failure was caused by the fact that the
>>>>>> __native_word() macro (used internally by compiletime_assert_atomic())
>>>>>> allows only a size of 4 or 8 for x86-64. The data type that I used is a
>>>>>> byte. Is there a reason why byte and short are not considered native?
>>>>>
>>>>> It seems likely it was implemented like that since there was no existing
>>>>> need; long can be relied on as the largest native type, so this should
>>>>> suffice and works here:
>>>>
>>>> There's Alphas that cannot actually atomically adres a byte; I do not
>>>> konw if Linux cares about them, but if it does, we cannot in fact rely
>>>> on this in generic primitives like this.
>>>
>>> That's right, and thanks for the heads-up. Alpha can only address 4
>>> and 8 bytes atomically. (LDL_L, LDQ_L, STL_C, STQ_C).
>>>
>>> The Byte-Word extension in EV56 doesn't add new atomics, so in fact no
>>> Alphas can address < 4 bytes atomically.
>>
>> Emulated with aligned 4 byte atomics, and masking.  The same is true for arm,
>> ppc, mips which, depending on cpu, also lack < 4 byte atomics.
>
> Which means that Alpha should be able to similarly emulate 1-byte and
> 2-byte atomics, correct?

If it's not possible to guarantee that GCC emits the 4-byte atomics by 
using a union, then we could emit the instructions via assembly. We'd 
introduce a macro to ensure lock word alignment, and this would be safe 
for unsigned counting up to the packed type limit. Maybe that's just too 
over-constrained though.

Thanks,
   Daniel
-- 
Daniel J Blueman
Principal Software Engineer, Numascale