Re: [PATCH v3] locking/qrwlock: Let qrwlock has same layout regardless of the endian

[xinhui <xinhui.pan@linux.vnet.ibm.com>]

On 2016年07月14日 03:54, Peter Zijlstra wrote:
> On Mon, Jun 20, 2016 at 02:20:52PM +0800, Pan Xinhui wrote:
>> This patch aims to get rid of endianness in queued_write_unlock(). We
>> want to set  __qrwlock->wmode to NULL, however the address is not
>> &lock->cnts in big endian machine. That causes queued_write_unlock()
>> write NULL to the wrong field of __qrwlock.
>>
>> Actually qrwlock can have same layout, IOW we can remove the #if
>> __little_endian in struct __qrwlock. With such modification, we only
>> need define some _QW* and _QR* with corresponding values in different
>> endian systems.
>>
>> Suggested-by: Will Deacon <will.deacon@arm.com>
>> Signed-off-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
>> Acked-by: Waiman Long <Waiman.Long@hpe.com>
>> ---
>
> Urgh, I hate this stuff :/
>
> OK, so I poked at this a bit and I ended up with the below; but now
> qrwlock and qspinlock are inconsistent; although I suspect qspinlock is
> similarly busted wrt endian muck.
>
> Not sure what to do..
>
Lets talk about the qspinlock.

for x86, We has already assumed that ->locked sit at the low 8 bits, as is
smp_store_release((u8 *)lock, 0);

Then we can do a favor, export ->locked but other fields as reserved.
say

struct __qspinlock_unlcok_interface {/* what name is better?*/
#ifdef __LITTLE_ENDIAN
		u8      locked;
		u8      reserved[3]; /* do not touch it, internally use only  */
#else
		u8      reserved[3];
		u8      locked;
#endif
};

I think it is acceptable. and we can do similar things with qrwlock, too.

any thoughts?


>   /*
> - * Writer states & reader shift and bias
> + * Writer states & reader shift and bias.
> + *
> + *       | +0 | +1 | +2 | +3 |
> + *   ----+----+----+----+----+
> + *    LE | 12 | 34 | 56 | 78 | 0x12345678
> + *   ----+----+----+----+----+
> + *    BE | 78 | 56 | 34 | 12 | 0x12345678
> + *   ----+----+----+----+----+
> + *       | wr |      rd      |
> + *       +----+----+----+----+
> + *
>    */

very clearly. :)

thanks
xinhui