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

[David Laight]

From: Boqun Feng
> Sent: 02 January 2024 18:54
> 
> On Sat, Dec 30, 2023 at 03:49:52PM +0000, David Laight wrote:
> [...]
> > But it looks odd that osq_unlock()'s fast path uses _release but the very
> > similar code in osq_wait_next() uses _acquire.
> >
> 
> The _release in osq_unlock() is needed since unlocks are needed to be
> RELEASE so that lock+unlock can be a critical section (i.e. no memory
> accesses can escape). When osq_wait_next() is used in non unlock cases,
> the RELEASE is not required. As for the case where osq_wait_next() is
> used in osq_unlock(), there is a xchg() preceding it, which provides a
> full barrier, so things are fine.

I know there have been issues with ACQUIRE/RELEASE/FULL xchg in this code,
but are FULL xchg always needed on node->next?

> /me wonders whether we can relax the _acquire in osq_wait_next() into
> a _relaxed.

I wouldn't have worried about relaxed v release.

> > 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;
> >
> 
> If so we need to provide some sort of RELEASE semantics for the
> osq_unlock() in all the cases.

I wonder how often the unqueue code happens, and especially for
the last cpu in the list?
I'd only expect need_resched() to return true after spinning for
a while - in which case perhaps it is more likely that there are
a lot of cpu in the queue and the cpu being removed won't be last.
So osq_wait_next() exits on xchg(&node->next, NULL) != NULL
which is full barrier.

On a slightly different note I've also wondered if 'osq_node'
actually needs to be cache line aligned?
You definitely don't want it spanning 2 cache line, but I'd
expect that per-cpu data is mostly accessed by its own cpu?
So you really aren't going to get false sharing with some
other per-cpu data since the cpu is busy in this code.
So __aligned(16) would do?

	David

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