Extract the spinlock from SkOnce as SkSpinlock.

Extract the spinlock from SkOnce as SkSpinlock.

This uses slightly newer APIs from SkAtomics.h to make it a bit more efficient.

No public API changes.
TBR=reed@google.com

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/50ffd9921e6dc82cae8ef143e8cecfd8bcf900fb

[mtklein_C, 2015-03-27 18:28:18]

mtklein@chromium.org changed reviewers:
+ joshualitt@google.com

[mtklein_C, 2015-03-27 18:28:18]

See if this still drops in as faster for your use case?

I'll follow up with the acquire() TODO in another CL.

[mtklein, 2015-03-27 19:02:39]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-03-27 19:02:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1039323002/1

[commit-bot: I haz the power, 2015-03-27 19:11:44]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-03-27 19:11:45]

This issue passed the CQ dry run.

[mtklein, 2015-03-27 20:31:54]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-03-27 20:32:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1039323002/20001

[commit-bot: I haz the power, 2015-03-27 20:55:41]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-03-27 20:55:41]

This issue passed the CQ dry run.

[joshualitt, 2015-03-27 21:36:12]

On 2015/03/27 20:55:41, I haz the power (commit-bot) wrote:
> This issue passed the CQ dry run.

definitely faster for our use case

[mtklein_C, 2015-03-27 22:04:23]

mtklein@chromium.org changed reviewers:
+ bungeman@google.com

[mtklein_C, 2015-03-27 22:04:24]

+Ben

This is what we were looking at before in person.  Mind taking a look?

[bungeman-skia, 2015-03-30 14:52:17]

I can find no useful fault, so lgtm.

https://codereview.chromium.org/1039323002/diff/40001/include/ports/SkAtomics...
File include/ports/SkAtomics_sync.h (right):

https://codereview.chromium.org/1039323002/diff/40001/include/ports/SkAtomics...
include/ports/SkAtomics_sync.h:64: } while(!sk_atomic_compare_exchange(ptr,
&prev, val));
nit: usually there is a space between the 'while' and the '('. A quick grep
shows that the usage like '} while(' in the codebase is far outnumbered by usage
like '} while ('. We should probably just change the style guide and the other
10 uses of '} while('.

[mtklein, 2015-03-30 14:57:47]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-03-30 14:58:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1039323002/40001

[commit-bot: I haz the power, 2015-03-30 15:13:39]

Committed patchset #3 (id:40001) as
https://skia.googlesource.com/skia/+/50ffd9921e6dc82cae8ef143e8cecfd8bcf900fb