Lock-Free Data Structures in C++ Compare and Swap Routine -

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in paper: lock-free data structures (pdf) following "compare , swap" fundamental shown:

template <class t> bool cas(t* addr, t exp, t val) {   if (*addr == exp)   {     *addr = val;     return true;   }   return false; }

and says

the entire procedure atomic

but how so? not possible other actor change value of addr between if , assignment? in case, assuming code using cas fundamental, found next time "expected" way, , wasn't. however, doesn't change fact happen, in case, still atomic? other actor returning true, when it's changes overwritten actor? if can't possibly happen, why?

i want believe author, missing here? thinking must obvious. apologies in advance if seems trivial.

he describing atomic operation given implementation, "somehow." pseudo-code implemented in hardware.


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