Messages in this thread | | | Date | Wed, 6 Jun 2018 13:59:19 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 0.5/5] alpha: remove custom dec_and_lock() implementation |
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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.
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