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    SubjectRe: [PATCH v9 04/19] qspinlock: Extract out the exchange of tail code word
    On 04/18/2014 04:15 AM, Peter Zijlstra wrote:
    > 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.

    For spin_lock(), the lock cacheline is touched by a cmpxchg(). It can
    takes 100s of cycles whether it is hot or cold.

    I will take the precheck out, it is not such a big deal anyway.

    -Longman


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