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Re: Synchronization of atomic notify and wait

From: Andrey Semashev <andrey.semashev_at_[hidden]>
Date: Tue, 19 Jul 2022 10:30:09 +0300
On 7/19/22 09:58, language.lawyer_at_[hidden] wrote:
> On 19/07/2022 11:43, Andrey Semashev via Std-Discussion wrote:
>> It means the store happens-before notify_one (in the non-main thread).
>> Coherence-ordered before is the relation between the store and the
>> wait in the two threads.
>> It says that when the store is coherence-ordered before the wait in
>> the modification order of the atomic, the wait will observe the
>> effects of the store.
> To me, http://eel.is/c++draft/atomics.order#3 reads in the opposite
> direction. If the wake-up read observes the new value, it will become
> coherence-ordered before. But if it does not observe, it won't.
> So, what guarantees it will observe the new value?

If the wait doesn't observe the store, it blocks. And in that case, it
unblocks when it observes the notify, which happens-after the store. So
if the wait is unblocked by notify, it must also observe the store on
the second load.

>> And since the store happens-before the notify, if the wait is woken by
>> the notify there is no way it doesn't observe the store.
> Because?..

I explained above why.

Received on 2022-07-19 07:30:13