Messages in this thread | | | Date | Mon, 14 Sep 2015 16:04:56 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt |
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On Fri, Sep 11, 2015 at 02:37:37PM -0400, Waiman Long wrote: > This patch allows one attempt for the lock waiter to steal the lock > when entering the PV slowpath. This helps to reduce the performance > penalty caused by lock waiter preemption while not having much of > the downsides of a real unfair lock.
> @@ -416,7 +414,8 @@ queue: > * does not imply a full barrier. > * > */
If it really were once, like the Changelog says it is, then you could have simply added:
if (pv_try_steal_lock(...)) goto release;
here, and not wrecked pv_wait_head() like you did. Note that if you do it like this, you also do not need to play games with the hash, because you'll never get into that situation.
> - pv_wait_head(lock, node); > + if (pv_wait_head_and_lock(lock, node, tail)) > + goto release; > while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_PENDING_MASK) > cpu_relax(); >
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