On 3/8/23 02:49, Paolo Bonzini wrote:
On 3/7/23 18:26, Richard Henderson wrote:
On 3/7/23 09:00, Paolo Bonzini wrote:
while QSLIST_REMOVE_HEAD in the dequeuing thread is not ordered at all:
y.store(0, mo_relaxed); // QSLIST_REMOVE_HEAD
x.store(0, mo_release); // fetch_and
As I read aio_bh_queue, this is exactly the situation you describe in patch 1
justifying the introduction of the new barriers.
Only store-store reordering is required between QSLIST_REMOVE_HEAD and
atomic_fetch_and(); and that one *is* blocked by atomic_fetch_and(), since mo_seq_cst
is a superset of both mo_release. The new barriers are needed for store-load reordering.
In patch 1 you say:
# in C11, sequentially consistent atomics (except for seq-cst fences)
# only affect the ordering of sequentially consistent operations.
and the store in QSLIST_REMOVE_HEAD is not a sequentially consistent operation.
Therefore by your own logic we must have a separate barrier here.
You're right that the comment is contradictory.
It's the comment that is wrong. The right text should be
---
in C11, with the exception of seq-cst fences, the order established by sequentially
consistent atomics does not propagate to other memory accesses on either side of the
seq-cst atomic. As far as those are concerned, loads performed by a seq-cst atomic are
just acquire loads, and stores are just release stores. Even though loads that occur
after a RMW operation cannot move above the load, they can still sneak above the store!
---
Ok, thanks.
I wonder if your definition/description of smp_mb__before_rmw() isn't actively
misleading in this case.
- We could drop it entirely and be less confusing, by not having to explain it.
- We could define it as signal_barrier() for all hosts, simply to fix the
compiler-theoretic reordering problem.
The case that I was imagining for smp_mb__before_rmw() is something like this:
wake_me = true;
smp_mb__before_rmw();
if (qatomic_xchg(&can_sleep, true)) { ... }
where you really need a full barrier.
What is different about this that doesn't apply to the remove-head case we've been talking
about?
r~
