Re: [PATCH 0.5/5] alpha: remove custom dec_and_lock() implementation

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jun 06, 2018 at 01:18:50PM +0200, Sebastian Andrzej Siewior wrote:

> - atomic_add_unless() adds more memory barriers compared to the custom
>   assembly

> I can't tell if the different memory barriers
> are an issue but having the same barriers is probably good.

refcount decrement needs to be a RELEASE operation, such that all the
load/stores to the object happen before we decrement the refcount.

Otherwise things like:

	obj-foo = 5;
	refcnt_dec(&obj->ref);

can be re-ordered, which then allows fun scenarios like:

	CPU0				CPU1

	refcnt_dec(&obj->ref);
					if (dec_and_test(&obj->ref))
						free(obj);
	obj->foo = 5; // oops UaF


This means (for alpha) that there should be a memory barrier _before_
the decrement, however the dec_and_lock asm thing only has one _after_,
which, per the above, is too late.

The generic version using add_unless will result in memory barrier
before and after (because that is the rule for atomic ops with a return
value) which is strictly too many barriers for the refcount story, but
who knows what other ordering requirements code has.