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We have now the situation that we cannot > switch off preemption support anymore. > > It seem that S390 has this mechanism in a small way and therefore can > avoid the preempt enable/disable. > > It is not the quantity of code here. The preempt enable/disable can only > be avoided if there is a single instruction doing the per cpu operation. A > single instruction cannot be interupted and therefore is preemption safe. > > >> That's a big difference to s390: for both primitives the result is a single >> instruction. > Ok then you can already use single instructions like x86 and will not have > preempt enable/disable overhead. I don't think S390 can do it in one single instruction. IIUC, Heiko means preempt_enable/disable is a single instruction on s390, but it is RMW on ARM64 (3 instructions for each). Thanks, Yang > > I am not sure what David Laight's code is supposed to do. Seems weird to > me. >