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

[Waiman Long]

On 10/02/2013 03:30 PM, Jason Low wrote:
> On Wed, Oct 2, 2013 at 12:19 PM, Waiman Long<waiman.long@xxxxxx>  wrote:
> > On 09/26/2013 06:42 PM, Jason Low wrote:
> > > On Thu, 2013-09-26 at 14:41 -0700, Tim Chen wrote:
> > > > Okay, that would makes sense for consistency because we always
> > > > first set node->lock = 0 at the top of the function.
> > > >
> > > > If we prefer to optimize this a bit though, perhaps we can
> > > > first move the node->lock = 0 so that it gets executed after the
> > > > "if (likely(prev == NULL)) {}" code block and then delete
> > > > "node->lock = 1" inside the code block.
> > > >
> > > > static noinline
> > > > void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node
> > > > *node)
> > > > {
> > > >          struct mcs_spin_node *prev;
> > > >
> > > >          /* Init node */
> > > >          node->next   = NULL;
> > > >
> > > >          prev = xchg(lock, node);
> > > >          if (likely(prev == NULL)) {
> > > >                  /* Lock acquired */
> > > >                  return;
> > > >          }
> > > >          node->locked = 0;
> >
> > You can remove the locked flag setting statement inside if (prev == NULL),
> > but you can't clear the locked flag after xchg(). In the interval between
> > xchg() and locked=0, the previous lock owner may come in and set the flag.
> > Now if your clear it, the thread will loop forever. You have to clear it
> > before xchg().
> Yes, in my most recent version, I left locked = 0 in its original
> place so that the xchg() can act as a barrier for it.
>
> The other option would have been to put another barrier after locked =
> 0. I went with leaving locked = 0 in its original place so that we
> don't need that extra barrier.

I don't think putting another barrier after locked=0 will work. 
Chronologically, the flag must be cleared before the node address is 
saved in the lock field. There is no way to guarantee that except by 
putting the locked=0 before xchg().

-Longman
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