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

[Paul E. McKenney]

On Wed, Jul 09, 2014 at 10:25:44AM +0930, Rusty Russell wrote:
> Tejun Heo <tj@xxxxxxxxxx> writes:
> > Hello, Paul.
> 
> Rusty wakes up...

;-)

> >> Good point.  How about per-CPU variables that are introduced by
> >> loadable modules?  (I would guess that there are plenty of memory
> >> barriers in the load process, given that text and data also needs
> >> to be visible to other CPUs.)
> >
> > (cc'ing Rusty, hi!)
> >
> > Percpu initialization happens in post_relocation() before
> > module_finalize().  There seem to be enough operations which can act
> > as write barrier afterwards but nothing seems explicit.
> >
> > I have no idea how we're guaranteeing that .data is visible to all
> > cpus without barrier from reader side.  Maybe we don't allow something
> > like the following?
> >
> >   module init				built-in code
> >
> >   static int mod_static_var = X;	if (builtin_ptr)
> >   builtin_ptr = &mod_static_var;		WARN_ON(*builtin_ptr != X);
> >
> > Rusty, can you please enlighten me?
> 
> Subtle, but I think in theory (though not in practice) this can happen.
> 
> Making this this assigner's responsibility is nasty, since we reasonably
> assume that .data is consistent across CPUs once code is executing
> (similarly on boot).
> 
> >> Again, it won't help for the allocator to strongly order the
> >> initialization to zero if there are additional initializations of some
> >> fields to non-zero values.  And again, it should be a lot easier to
> >> require the smp_store_release() or whatever uniformly than only in cases
> >> where additional initialization occurred.
> >
> > This one is less murky as we can say that the cpu which allocated owns
> > the zeroing; however, it still deviates from requiring the one which
> > makes changes to take care of barriering for those changes, which is
> > what makes me feel a bit uneasy.  IOW, it's the allocator which
> > cleared the memory, why should its users worry about in-flight
> > operations from it?  That said, this poses a lot less issues compared
> > to percpu ones as passing normal pointers to other cpus w/o going
> > through proper set of barriers is a special thing to do anyway.
> 
> I think that the implicit per-cpu allocations done by modules need to
> be consistent once the module is running.
> 
> I'm deeply reluctant to advocate it in the other per-cpu cases though.
> Once we add a barrier, it's impossible to remove: callers may subtly
> rely on the behavior.
> 
> "Magic barrier sprinkles" is a bad path to start down, IMHO.

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.

Thoughts?

							Thanx, Paul

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