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

From: Jason Low
Date: Fri Sep 27 2013 - 19:54:18 EST

On Fri, Sep 27, 2013 at 4:01 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> 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@xxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
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 */
ACCESS_ONCE(prev->next) = node;
- smp_wmb();
/* Wait until the lock holder passes the lock down */
while (!ACCESS_ONCE(node->locked))
+ 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)))
- ACCESS_ONCE(next->locked) = 1;
+ ACCESS_ONCE(next->locked) = 1;

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