On Tue, Jun 14, 2016 at 02:11:48PM +0800, xinhui wrote:

On 2016年06月08日 17:22, Will Deacon wrote:

quoted

On Thu, Jun 02, 2016 at 06:09:08PM +0800, Pan Xinhui wrote:

quoted

strcut __qrwlock has different layout in big endian machine. we need set
the __qrwlock->wmode to NULL, and the address is not &lock->cnts in big
endian machine.

Do as what read unlock does. we are lucky that the __qrwlock->wmode's
val is _QW_LOCKED.

Doesn't this have wider implications for the qrwlocks, for example:

  while ((cnts & _QW_WMASK) == _QW_LOCKED) { ... }

would actually end up looking at the wrong field of the lock?

I does not clearly understand your idea. :(

That's because I'm talking rubbish :) Sorry, I completely confused myself.
Locking is bad enough on its own, but add big-endian to the mix and I'm
all done.

quoted

Shouldn't we just remove the #ifdef __LITTLE_ENDIAN stuff from __qrwlock,
given that all the struct members are u8?

No. that makes codes complex. for example

struct __qrwlock lock;

WRITE_ONCE(lock->wmode, _QW_WAITING);
if (atomic_(&lock->cnts) == _QW_WAITING) {
	do_something();
}

IF you remove the  #ifdef __LITTLE_ENDIAN stuff from __qrwlock.
codes above obviously will break. And we already have such code.

I was wondering more along the lines of having one definition of the data
structure, but then defining _QW_* differently depending on endianness
(i.e. add a << 24 when big-endian). That way queued_write_unlock can
stay like it is (having an arch override to handle the big-endian case
is incredibly ugly).

Will