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In the generic atomic ops that rely on a loop around a CAS primitive, there's no need to force the initial read of the "old" value to be atomic. In the typically-rare case that we get a torn value, that simply means that the first CAS attempt will fail; but it will update "old" to the atomically-read value, so the next attempt has a chance of succeeding. It was already being done that way in pg_atomic_exchange_u64_impl(), but let's duplicate the approach in the rest. (Given the current coding of the pg_atomic_read functions, this change is a no-op anyway on popular platforms; it only makes a difference where pg_atomic_read_u64_impl() is implemented as a CAS.) In passing, also remove unnecessary take-a-pointer-and-dereference-it coding in the pg_atomic_read functions. That seems to have been based on a misunderstanding of what the C standard requires. What actually matters is that the pointer be declared as pointing to volatile, which it is. I don't believe this will change the assembly code at all on x86 platforms (even ignoring the likelihood that these implementations get overridden by others); but it may help on less-mainstream CPUs. Discussion: https://postgr.es/m/13707.1504718238@sss.pgh.pa.us
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