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

Jason Low
2013-09-28T04:34:10Z

On Fri, Sep 27, 2013 at 7:19 PM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> 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. ;-)

Sure. Here is a link to the patch we are trying to modify:
https://lkml.org/lkml/2013/9/25/532

Also, below is what the mcs_spin_lock() and mcs_spin_unlock()
functions would look like after applying the proposed changes.

static noinline
void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node *node)
{
struct mcs_spin_node *prev;

/* Init node */
node->locked = 0;
node->next = NULL;

prev = xchg(lock, node);
if (likely(prev == NULL)) {
/* Lock acquired. No need to set node->locked since it
won't be used */
return;
}
ACCESS_ONCE(prev->next) = node;
/* 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)
{
struct mcs_spin_node *next = ACCESS_ONCE(node->next);

if (likely(!next)) {
/*
* Release the lock by setting it to NULL
*/
if (cmpxchg(lock, node, NULL) == node)
return;
/* Wait until the next pointer is set */
while (!(next = ACCESS_ONCE(node->next)))
arch_mutex_cpu_relax();
}
smp_wmb();
ACCESS_ONCE(next->locked) = 1;
}