On Thu, Apr 17, 2014 at 05:28:17PM -0400, Waiman Long wrote: > 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.
But you did that read _before_ you touched a cold cacheline, that's 100s of cycles. Whatever value you read back then is now complete nonsense. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
