RE: [PATCH next 2/5] locking/osq_lock: Avoid dirtying the local cpu's 'node' in the osq_lock() fast path.

[David Laight]

From: Waiman Long
> Sent: 30 December 2023 03:20
> 
> On 12/29/23 17:11, David Laight wrote:
> > osq_lock() starts by setting node->next to NULL and node->locked to 0.
> > Careful analysis shows that node->next is always NULL on entry.
> >
> > node->locked is set non-zero by another cpu to force a wakeup.
> > This can only happen after the 'prev->next = node' assignment,
> > so locked can be set to zero just before that (along with the assignment
> > to node->prev).
> >
> > Only initialise node->cpu once, after that use its value instead
> > of smp_processor_id() - which is probably a real function call.
> >
> > Should reduce cache-line bouncing a little.
> >
> > Signed-off-by: David Laight <david.laight@xxxxxxxxxx>
> > ---
> >
> > Re-send without the 'RE:' on the subject line.
> >   kernel/locking/osq_lock.c | 13 ++++++-------
> >   1 file changed, 6 insertions(+), 7 deletions(-)
> >
> > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> > index d414eef4bec6..55f5db896c02 100644
> > --- a/kernel/locking/osq_lock.c
> > +++ b/kernel/locking/osq_lock.c
> > @@ -51,7 +51,7 @@ osq_wait_next(struct optimistic_spin_queue *lock,
> >   	      struct optimistic_spin_node *prev)
> >   {
> >   	struct optimistic_spin_node *next = NULL;
> > -	int curr = encode_cpu(smp_processor_id());
> > +	int curr = node->cpu;
> >   	int old;
> >
> >   	/*
> > @@ -98,12 +98,10 @@ bool osq_lock(struct optimistic_spin_queue *lock)
> >   {
> >   	struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
> >   	struct optimistic_spin_node *prev, *next;
> > -	int curr = encode_cpu(smp_processor_id());
> >   	int old;
> >
> > -	node->locked = 0;
> > -	node->next = NULL;
> 
> I have some concern about not clearing node->next at the beginning. I
> know that next is supposed to be cleared at the end. I do have some
> worry that there may exist a race condition that leaves next not cleared
> before it is used again. So you either have to prove that such a race
> does not exist, or initializing it to NULL at the beginning like it is
> today.

I've stared at the code some more :-)

There are two places where node->next is written non-NULL, both in osq_lock().
The first is at the top of the slow-path where prev->next = node
(this should be overwriting the NULL set (or not) on entry).
The second is at the bottom of osq_lock() prev->next = next (Step C)
where the NULL written in Step A is written with the correct 'next' node.
After either of those the 'node' cpu must later either take the unqueue
exit from osq_lock() or call osq_unlock().
Both require it wait for node->next be non-NULL and NULL it.

If code calls osq_lock() twice all bets are off!

Even if (somehow) node->next was non-NULL on entry it will be set by an
osq_lock() call from another cpu.
The only problem would be if osq_unlock() were called before the queueing
cpu set prev->next = node.
That in itself is unlikely - but would happen if node->next were always set.

I don't completely understand the 'acquire'/'release' semantics (they didn't
exist when I started doing SMP kernel code in the late 1980s).
But it looks odd that osq_unlock()'s fast path uses _release but the very
similar code in osq_wait_next() uses _acquire.

Indeed, apart from some (assumed) optimisations, I think osq_unlock()
could just be:
	next = osq_wait_next(lock, this_cpu_ptr(&osq_node), 0);
	if (next)
		next->locked = 1;

I don't think the order of the tests for lock->tail and node->next
matter in osq_wait_next().
If they were swapped the 'Second most likely case' code from osq_unlock()
could be removed.
(The 'uncontended case' doesn't need to load the address of 'node'.)

	David
		

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