Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file

Davidlohr Bueso
2013-09-28T00:02:40Z

On Fri, 2013-09-27 at 16:54 -0700, Jason Low wrote:
> On Fri, Sep 27, 2013 at 4:01 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > Yep. The previous lock holder's smp_wmb() won't keep either the compiler
> > or the CPU from reordering things for the new lock holder. They could for
> > example reorder the critical section to precede the node->locked check,
> > which would be very bad.
>
> Paul, Tim, Longman,
>
> How would you like the proposed changes below?
>
> ---
> Subject: [PATCH] MCS: optimizations and barrier corrections

We *really* need to comment those barriers - explicitly that is :)

>
> Delete the node->locked = 1 assignment if the lock is free as it won't be used.
>
> Delete the smp_wmb() in mcs_spin_lock() and add a full memory barrier at the
> end of the mcs_spin_lock() function. As Paul McKenney suggested, "you do need a
> full memory barrier here in order to ensure that you see the effects of the
> previous lock holder's critical section." And in the mcs_spin_unlock(), move the
> memory barrier so that it is before the "ACCESS_ONCE(next->locked) = 1;".
>
> Signed-off-by: Jason Low <jason.low2@hp.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> ---
> include/linux/mcslock.h | 7 +++----
> 1 files changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/mcslock.h b/include/linux/mcslock.h
> index 20fd3f0..edd57d2 100644
> --- a/include/linux/mcslock.h
> +++ b/include/linux/mcslock.h
> @@ -26,15 +26,14 @@ void mcs_spin_lock(struct mcs_spin_node **lock,
> struct mcs_spin_node *node)
>
> prev = xchg(lock, node);
> if (likely(prev == NULL)) {
> - /* Lock acquired */
> - node->locked = 1;
> + /* Lock acquired. No need to set node->locked since it
> won't be used */

Then, we need to explain/comment then the relationship between this
situation and the locked being set in mspin_unlock(), passing the lock
holder down the list.

> return;
> }
> ACCESS_ONCE(prev->next) = node;
> - smp_wmb();
> /* Wait until the lock holder passes the lock down */
> while (!ACCESS_ONCE(node->locked))
> arch_mutex_cpu_relax();
> + smp_mb();
> }
>
> static void mcs_spin_unlock(struct mcs_spin_node **lock, struct
> mcs_spin_node *node)
> @@ -51,8 +50,8 @@ static void mcs_spin_unlock(struct mcs_spin_node
> **lock, struct mcs_spin_node *n
> while (!(next = ACCESS_ONCE(node->next)))
> arch_mutex_cpu_relax();
> }
> - ACCESS_ONCE(next->locked) = 1;
> smp_wmb();
> + ACCESS_ONCE(next->locked) = 1;
> }
>
> #endif