On Fri, May 09, 2014 at 08:58:47PM -0400, Waiman Long wrote:
> On 05/08/2014 02:58 PM, Peter Zijlstra wrote:
> >On Wed, May 07, 2014 at 11:01:34AM -0400, Waiman Long wrote:
> >>@@ -221,11 +222,37 @@ static inline int trylock_pending(struct qspinlock *lock, u32 *pval)
> >> */
> >> for (;;) {
> >> /*
> >>- * If we observe any contention; queue.
> >>+ * If we observe that the queue is not empty,
> >>+ * return and be queued.
> >> */
> >>- if (val& ~_Q_LOCKED_MASK)
> >>+ if (val& _Q_TAIL_MASK)
> >> return 0;
> >>
> >>+ if (val == (_Q_LOCKED_VAL|_Q_PENDING_VAL)) {
> >>+ /*
> >>+ * If both the lock and pending bits are set, we wait
> >>+ * a while to see if that either bit will be cleared.
> >>+ * If that is no change, we return and be queued.
> >>+ */
> >>+ if (!retry)
> >>+ return 0;
> >>+ retry--;
> >>+ cpu_relax();
> >>+ cpu_relax();
> >>+ *pval = val = atomic_read(&lock->val);
> >>+ continue;
> >>+ } else if (val == _Q_PENDING_VAL) {
> >>+ /*
> >>+ * Pending bit is set, but not the lock bit.
> >>+ * Assuming that the pending bit holder is going to
> >>+ * set the lock bit and clear the pending bit soon,
> >>+ * it is better to wait than to exit at this point.
> >>+ */
> >>+ cpu_relax();
> >>+ *pval = val = atomic_read(&lock->val);
> >>+ continue;
> >>+ }
> >Didn't I give a much saner alternative to this mess last time?
>
> I don't recall you have any suggestion last time. Anyway, if you think the
> code is too messy, I think I can give up the first if statement which is
> more an optimistic spinning kind of code for short critical section. The 2nd
> if statement is still need to improve chance of using this code path due to
> timing reason. I will rerun my performance test to make sure it won't have
> too much performance impact.
lkml.kernel.org/r/20140417163640.GT11096@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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