Re: [RFC] arm64: Enforce observed order for spinlock and data

[Mark Rutland]

Hi Brent,

Could you *please* clarify if you are trying to solve:

(a) a correctness issue (e.g. data corruption) seen in practice.
(b) a correctness issue (e.g. data corruption) found by inspection.
(c) A performance issue, seen in practice.
(d) A performance issue, found by inspection.

Any one of these is fine; we just need to know in order to be able to
help effectively, and so far it hasn't been clear.

On Tue, Oct 04, 2016 at 01:53:35PM -0400, bdegraaf@xxxxxxxxxxxxxx wrote:
> After looking at this, the problem is not with the lockref code per
> se: it is a problem with arch_spin_value_unlocked(). In the
> out-of-order case, arch_spin_value_unlocked() can return TRUE for a
> spinlock that is in fact locked but the lock is not observable yet via
> an ordinary load. 

Given arch_spin_value_unlocked() doesn't perform any load itself, I
assume the ordinary load that you are referring to is the READ_ONCE()
early in CMPXCHG_LOOP().

It's worth noting that even if we ignore ordering and assume a
sequentially-consistent machine, READ_ONCE() can give us a stale value.
We could perform the read, then another agent can acquire the lock, then
we can move onto the cmpxchg(), i.e.

    CPU0                              CPU1
    old = READ_ONCE(x.lock_val)
                                      spin_lock(x.lock)
    cmpxchg(x.lock_val, old, new)
                                      spin_unlock(x.lock)

If the 'old' value is stale, the cmpxchg *must* fail, and the cmpxchg
should return an up-to-date value which we will then retry with.

> Other than ensuring order on the locking side (as the prior patch
> did), there is a way to make arch_spin_value_unlock's TRUE return
> value deterministic, 

In general, this cannot be made deterministic. As above, there is a race
that cannot be avoided.

> but it requires that it does a write-back to the lock to ensure we
> didn't observe the unlocked value while another agent was in process
> of writing back a locked value.

The cmpxchg gives us this guarantee. If it successfully stores, then the
value it observed was the same as READ_ONCE() saw, and the update was
atomic.

There *could* have been an intervening sequence between the READ_ONCE
and cmpxchg (e.g. put(); get()) but that's not problematic for lockref.
Until you've taken your reference it was possible that things changed
underneath you.

Thanks,
Mark.