Re: [PATCH RFC] percpu: add data dependency barrier in percpu accessors and operations

[Christoph Lameter <cl@gentwo.org>]

On Mon, 14 Jul 2014, Paul E. McKenney wrote:

> Here is the sort of thing that I would be concerned about:
>
> 	p = alloc_percpu(struct foo);
> 	for_each_possible_cpu(cpu)
> 		initialize(per_cpu_ptr(p, cpu);
> 	gp = p;
>
> We clearly need a memory barrier in there somewhere, and it cannot
> be buried in alloc_percpu().  Some cases avoid trouble due to locking,
> for example, initialize() might acquire a per-CPU lock and later uses
> might acquire that same lock.  Clearly, use of a global lock would not
> be helpful from a scalability viewpoint.

The knowledge about the offset p is not available before gp is assigned
to.

gp usually is part of a struct that contains some form of serialization.
F.e. in the slab allocators there is a kmem_cache structure that contains
gp.

After alloc_percpu() and other preparatory work the structure is inserted
into a linked list while holding the global semaphore (slab_mutex). After
release of the semaphore the kmem_cache address is passed to the
subsystem. Then other processors can potentially use that new kmem_cache
structure to access new percpu data related to the new cache.

There is no scalability issue for the initialization since there cannot
be a concurrent access since the offset of the percpu value is not known
by other processors at that point.