Pthread spinlock support

[valtteri at rahkonen.fi]

Hi,

I noticed that the OSX version of the jemalloc uses spin locks and decided 
to implement support for the pthread spin locks that can be used in Linux. 
At least in my case there is huge benefit because I run a single thread in 
a specific core that has not that much other activity and pthread mutex 
lock contention seems to always schedule the thread away from execution, 
so spin locking seems to give more stable result while adding bit more CPU 
load. Most likely in more general case this would not be wanted, because 
there would be more threads/processes running on same core and thus the 
scheduling might give execution time for some other important threads like 
the one having the lock taken.

What do you think, is this something that could be merged to the upstream? 
My patch implements new configure script option --enable-pthread-spinlock 
that turns on the pthread spin lock i.e. the spin locks are not used by 
default because of the scheduling benefit with normal use.

Best regards,
Valtteri
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