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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Sep 27, 2013 at 04:54:06PM -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?

Could you point me at what this applies to?  I can find flaws looking
at random pieces, given a little luck, but at some point I need to look
at the whole thing.  ;-)

							Thanx, Paul

> ---
> 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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