[Bug 265974] SMR has several missing barriers

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=265974

--- Comment #9 from Pierre Habouzit <pierre@habouzit.net> ---
`atomic_thread_fence_rel()` is a `dmb sy` ? I would have expected that to be a
`dmb st` ..

but indeed it shows I'm not familiar with with FreeBSD:

```C
static __inline void
atomic_thread_fence_rel(void)
{

        dmb(sy);
}
```

and indeed it makes the deferred advance correct then.

It also means smr_synchronize() has a stronger semantics than I expected
already and makes the smr_poll barrier on entry useless because the
`atomic_load_acq_int()` in smr_poll() already give the semantics I was after on
entry.

I think you might be right that the one on exit of smr_poll() isn't quite
needed. I think I just didn't think it through and just took
`ck_epoch_synchronize_wait` as an example and just was like "it needs one so
smr_scan does too" but trying to come up with a reason why it breaks yields
nothing..

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