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

[Jason Low <jason.low2@hp.com>]

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

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 */
                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
-- 
1.7.1

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