TSX lock elision for glibc v12

[Andi Kleen]

> To clarify, with my approach, the ONLY cases where applications would
> get the no-elision penalty is when they explicitly create a
> normal-type mutex or call pthread_mutexattr_settype without using the
> new version of pthread.h. Using PTHREAD_MUTEX_INITIALIZER or
> pthread_mutex_init without calling pthread_mutexattr_settype on the
> attribute (or with a null attribute) would result in a default-type
> mutex and thus would allow elision.
> 
> I think this approach would make even Andi happy. :-)

To make sure I understand your approach correctly:

- Have a new ABI version of pthread_mutexattr_settype()
- Define a new PTHREAD_MUTEX_NORMAL enum
- The new ABI version of settype() would understand a
new different value for PTHREAD_MUTEX_NORMAL and if that
value is set, set either an additional disable elision
or disable nested non trylock elision flag.
It would enable elision for passing 0.
- The old ABI version of settype would always set the 
no elision or no nested elision flags for the current 0 value
(aka TIMED/NORMAL/DEFAULT)
- Just using 0 in an initializer will not set any of the
additional flags that disable elision.

Is that correct?

I have to check it out a bit more first and do some grepping/testing,
but yes that approach could satisfy my compatibility requirements.

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only.