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

[Tejun Heo]

Hello, Paul.

On Tue, Jun 17, 2014 at 07:41:51AM -0700, Paul E. McKenney wrote:
> > Well, we can always say that it's the caller's responsibility to put
> > in enough barriers, but it becomes weird for percpu areas because the
> > ownership of the allocated areas isn't really clear.  Whether each
> > allocated cpu-specific area's queued writes belong to the allocating
> > cpu or the cpu that the area is associated with is an implementation
> > detail which may theoretically change and as such I think it'd be best
> > for the problem to be dealt with in percpu allocator and accessors
> > proper.  I think this is way too subtle to ask the users to handle
> > correctly.
> 
> But in the case where the allocation is initially zeroed, then a few
> fields are initialized to non-zero values, the memory barrier in the
> allocator is insufficient.  I believe that it will be easier for people

Oh, definitely but in those cases it's clear whose responsbility it
is.  The task/cpu which modified the memory owns the in-flight
modifications and is responsible for propagation.  The problem with
zeroed allocation, especially for percpu areas, is that it isn't clear
who owns the zeroing.  Again, there's no reason the zeroing can't
belong to the each local cpu, which could have performance benefits if
we ever get frequent enough with percpu allocations.  To me, this
really seems like an implementation detail which shouldn't leak to the
users of the allocator.

> to always use smp_store_release() or whatever than to not need it if
> the initialization is strictly zero, but to need it if additional
> initialization is required.

That goes further and would require dedicated assignment macros for
percpu pointers, a la rcu_assign_pointer().  Yeah, I think that would
be helpful.  It'd take quite a bit of work but percpu allocator usage
is still in low hundreds, so it should be doable.  The whole thing is
way too subtle to require each user to handle it manually.

So, how about the followings?

* Add data dependency barrier to percpu accessors and write barrier to
  the allocator with the comment that this will be replaced with
  proper assignment macros and mark this change w/ -stable.

* Later, introduce percpu pointer assignment macros and convert all
  users and remove the wmb added above.

> > > I was worried about use of per_cpu() by the reading CPU, but of course
> > > in that case, things are initialized at compiler time.
> > 
> > Not really.  The template is initialized on kernel load.  The actual
> > percpu areas have to be allocated dynamically; however, this happens
> > while the machine is still running UP, so this should be okay.
> 
> 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?

> > For normal allocations, I don't have a strong opinion.  I'd prefer to
> > think of memory coming out of the allocator to have memory ordering
> > already taken care of but it is a lot less murky than with percpu
> > areas.
> 
> 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.

Thanks.

-- 
tejun
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