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atomic_compare_exchange atomicity of write to *expected

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For the atomic_compare-exchange family, e.g.

_Bool atomic_compare_exchange_strong(volatile A *object, C *expected, C
desired);

the description states "Atomically, compares the contents of the memory
pointed to by object for equality with that pointed to by expected, and
if true, replaces the contents of the memory pointed to by object with
desired, and if false, updates the contents of the memory pointed to by
expected with that pointed to by object."

This reads to me as if the write to *expected is also part of the atomic
operation. This doesn't map well to compare-and-swap hardware
instructions, where the content of *object would be written to a
register, and later stored into *expected by a separate instruction.

Of course, this difference only matters if it is observable. I think it
is observable if just after the compare-and swap, a second thread writes
*object, while the first thread has already done the comparison, but not
yet updated *expected.
Then e.g. a thread thread could read the new value from *object, but the
old one from *expected, which is not possible if
atomic_compare_exchange_strong is fully atomic (i.e. also wrt. *expected).

Is my understanding here correct (I'm not an expert on atomics)?

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atomic_compare_exchange atomicity of write to *expected Philipp Klaus Krause <pkk@spth.de> - 2021-10-05 12:46 +0200
  Re: atomic_compare_exchange atomicity of write to *expected Tijl Coosemans <tijl@coosemans.org> - 2021-10-08 13:20 +0200
    Re: atomic_compare_exchange atomicity of write to *expected Florian Weimer <fw@deneb.enyo.de> - 2021-10-11 07:51 +0200
      Re: atomic_compare_exchange atomicity of write to *expected Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-12-18 05:50 -0800
  Re: atomic_compare_exchange atomicity of write to *expected Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-10-09 03:04 -0700

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