On 2016年06月27日 15:55, Peter Zijlstra wrote:
On Sun, Jun 26, 2016 at 12:59:01PM +0800, panxinhui wrote:
在 2016年6月26日，03:12，Peter Zijlstra <pet...@infradead.org> 写道：
On Sun, Jun 26, 2016 at 01:27:51AM +0800, panxinhui wrote:
by the way I still think mutex_unlock has a big overload too.
Do you mean overhead?
oh, maybe you are right.
mutex_unlock ’s implementation uses inc_return variant on ppc, and
that’s expensive. I am thinking of using cmpxchg instead.
That statement doesn't make any sense. PPC is an LL/SC arch, inc_return
and cmpxchg are the 'same' LL/SC loop.
This is a little optimize.
if there are lock waiters, the lockval is minus X, when we call unlock, it will
inc the lockval, if it is <= 0, enter unlockslowpath to wakeup the waiters, and
set lockval to 1 in the slowpath.
SO there is no need to inc lockval if it is already a minus number. therefore
we can save one store or loads/stores in LL/SC loops
the base idea is from code below,
if (!atomic_read(&lk)//no need to call atomic_inc_return which is expensive.
atomic_inc_return(&lk))
