On Fri, Nov 08, 2013 at 11:52:05AM -0800, Tim Chen wrote:
> From: Jason Low <[email protected]>
>
> Remove unnecessary operation and make the cmpxchg(lock, node, NULL) == node
> check in mcs_spin_unlock() likely() as it is likely that a race did not occur
> most of the time.
>
> Also add in more comments describing how the local node is used in MCS locks.
>
> Reviewed-by: Tim Chen <[email protected]>
> Signed-off-by: Jason Low <[email protected]>
> ---
> include/linux/mcs_spinlock.h | 13 +++++++++++--
> 1 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/mcs_spinlock.h b/include/linux/mcs_spinlock.h
> index b5de3b0..96f14299 100644
> --- a/include/linux/mcs_spinlock.h
> +++ b/include/linux/mcs_spinlock.h
> @@ -18,6 +18,12 @@ struct mcs_spinlock {
> };
>
> /*
> + * In order to acquire the lock, the caller should declare a local node and
> + * pass a reference of the node to this function in addition to the lock.
> + * If the lock has already been acquired, then this will proceed to spin
> + * on this node->locked until the previous lock holder sets the node->locked
> + * in mcs_spin_unlock().
> + *
> * We don't inline mcs_spin_lock() so that perf can correctly account for the
> * time spent in this lock function.
> */
> @@ -33,7 +39,6 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct
> mcs_spinlock *node)
> prev = xchg(lock, node);
> if (likely(prev == NULL)) {
> /* Lock acquired */
> - node->locked = 1;
Agreed, no one looks at this field in this case, so no need to initialize
it, unless for debug purposes.
> return;
> }
> ACCESS_ONCE(prev->next) = node;
> @@ -43,6 +48,10 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct
> mcs_spinlock *node)
> arch_mutex_cpu_relax();
> }
>
> +/*
> + * Releases the lock. The caller should pass in the corresponding node that
> + * was used to acquire the lock.
> + */
> static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock
> *node)
> {
> struct mcs_spinlock *next = ACCESS_ONCE(node->next);
> @@ -51,7 +60,7 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock,
> struct mcs_spinlock *nod
> /*
> * Release the lock by setting it to NULL
> */
> - if (cmpxchg(lock, node, NULL) == node)
> + if (likely(cmpxchg(lock, node, NULL) == node))
Agreed here as well. Takes a narrow race to hit this.
So, did your testing exercise this path? If the answer is "yes", and
if the issues that I called out in patch #1 are resolved:
Reviewed-by: Paul E. McKenney <[email protected]>
> return;
> /* Wait until the next pointer is set */
> while (!(next = ACCESS_ONCE(node->next)))
> --
> 1.7.4.4
>
>
>
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