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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Jul 14, 2014 at 10:22:08AM -0500, Christoph Lameter wrote:
> 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.

If I understand your initialization procedure correctly, you need at least
an smp_wmb() on the update side and at least an smp_read_barrier_depends()
on the read side.

							Thanx, Paul