Re: [PATCH] Convert powerpc simple spinlocks into ticket locks

On Thu, Feb 06, 2014 at 11:37:37AM +0100, Torsten Duwe wrote:

x86 has them, MIPS has them, ARM has them, even ia64 has them:
ticket locks. They reduce memory bus and cache pressure especially
for contended spinlocks, increasing performance.

This patch is a port of the x86 spin locks, mostly written in C,
to the powerpc, introducing inline asm where needed. The pSeries
directed yield for vCPUs is taken care of by an additional "holder"
field in the lock.

A few questions; what's with the ppc64 holder thing? Not having a 32bit
spinlock_t is sad.

Can you pair lwarx with sthcx ? I couldn't immediately find the answer
in the PowerISA doc. If so I think you can do better by being able to
atomically load both tickets but only storing the head without affecting
the tail.

In that case you can avoid the ll/sc on unlock, because only the lock
owner can modify the tail, so you can use a single half-word store.