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Re: [Qemu-devel] [PATCH v1 3/5] include/qemu/atomic.h: default to __atom
Re: [Qemu-devel] [PATCH v1 3/5] include/qemu/atomic.h: default to __atomic functions
Mon, 4 Apr 2016 23:35:40 -0400
On Mon, Apr 4, 2016 at 1:03 PM, Paolo Bonzini <address@hidden> wrote:
>> "Establishes memory synchronization ordering of non-atomic and relaxed
>> atomic accesses"
> You can find some information at
> https://bugzilla.redhat.com/show_bug.cgi?id=1142857 and
This bug seems to be out-of-bounds. I get a message saying that the
bug has been restricted for internal development processes.
> I've looked at private email from that time and I was pointed to this
> sentence in GCC's manual, which says the opposite:
> "Note that in the C++11 memory model, fences (e.g.,
> ‘__atomic_thread_fence’) take effect in combination with other
> atomic operations on specific memory locations (e.g., atomic loads);
> operations on specific memory locations do not necessarily affect
> other operations in the same way."
> Basically, the __atomic fences are essentially a way to break up a
> non-relaxed atomic access into a relaxed atomic access and the
> ordering-constraining fence. According to those emails, atomic
> load-acquires and store-releases can provide synchronization for plain
> loads and stores (not just for relaxed atomic loads and stores).
> However, an atomic thread fence cannot do this.
Yes, on further research... this looks to be the case. I think I
understand the reasoning for this is to separate synchronization
variables/objects vs data.
> It depends on the compiler you're using. With some luck, reverting the
> patch will cause accesses across smp_wmb() or smp_rmb() to get
> reordered. In our case it was in thread-pool.c; Kevin Wolf did the
If ordering was crucial for those stores, I think it would have been
better to use atomics on them to enforce ordering.
Also, do you plan to introduce load_acquire/store_release() operations
like done in the linux kernel? They would cleanly map to gcc atomics
and make the ordering requirements explicit.