Messages in this thread | | | Date | Thu, 17 Apr 2014 17:28:17 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v9 04/19] qspinlock: Extract out the exchange of tail code word |
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On 04/17/2014 11:49 AM, Peter Zijlstra wrote: > On Thu, Apr 17, 2014 at 11:03:56AM -0400, Waiman Long wrote: >> @@ -192,36 +220,25 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 val) >> node->next = NULL; >> >> /* >> + * We touched a (possibly) cold cacheline; attempt the trylock once >> + * more in the hope someone let go while we weren't watching as long >> + * as no one was queuing. >> */ >> + if (!(val& _Q_TAIL_MASK)&& queue_spin_trylock(lock)) >> + goto release; > But you just did a potentially very expensive op; @val isn't > representative anymore!
That is not true. I pass in a pointer to val to trylock_pending() (the pointer thing) so that it will store the latest value that it reads from the lock back into val. I did miss one in the PV qspinlock exit loop. I will add it back when I do the next version.
-Longman
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