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. -- To unsubscribe from this list: send the line "unsubscribe linux-alpha" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
