On Fri, 4 Dec 2009, Phil Endecott wrote:
Helge Bahmann wrote:
there is a pseudo-code explanation in the boost::atomic class documentation
Ah sorry, I didn't realise that compare_exchange_* were in the external interface; for some reason I thought they were internal.
they are external, but I have now changed the implementation that backends must implement "four-operand" compare_exchange_*, which specifies two memory_order operands (one for successful exchange, one for failed), but give it a day until it is committed into the public repo the three-operand version is then provided by the front-end
OK, so since my kernel_cmpxchg() function doesn't return the old value but only a flag, I need something like:
bool success = kernel_cmpxchg(e,d,&i); if (!success) e = i;
But i may have changed again between the kernel_cmpxchg() and that assignment. Is that OK?
that's okay -- there are two cases: - "new" value (loaded after failed cmpxchg) does not match "expected", then the caller receive a more up-to-date value and retry with this one, after all it does not matter whether it failed with the "old" or "new" mismatching value - "new" value (loaded after failed cmpxchg) matches "expected": compare_exchange_weak is allowed to fail "spuriously"
Should I use load() there? If so, what memory order is needed?
the load is internal, so load it "relaxed" , you only have to maintain the memory order for the "total" cmpxchg, successful or failed Best regards, Helge