On Tue, Sep 22, 2015 at 04:50:43PM -0400, Waiman Long wrote:
> for (;; waitcnt++) {
> + loop = SPIN_THRESHOLD;
> + while (loop) {
> + /*
> + * Spin until the lock is free
> + */
> + for (; loop && READ_ONCE(l->locked); loop--)
> + cpu_relax();
> + /*
> + * Seeing the lock is free, this queue head vCPU is
> + * the rightful next owner of the lock. However, the
> + * lock may have just been stolen by another task which
> + * has entered the slowpath. So we need to use atomic
> + * operation to make sure that we really get the lock.
> + * Otherwise, we have to wait again.
> + */
> + if (cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0)
> + goto gotlock;
> }
for (loop = SPIN_THRESHOLD; loop; --loop) {
if (!READ_ONCE(l->locked) &&
cmpxchg(&l->locked, 0, _Q_LOCKED_VA) == 0)
goto gotlock;
cpu_relax();
}
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