Messages in this thread Patch in this message | | | Date | Fri, 27 Sep 2013 16:54:06 -0700 | Subject | Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file | From | Jason Low <> |
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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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