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> preempt_disable_count = this_cpu_data->preempt_disable_count; > If the count was non-zero at the start you'll read the value from > the current cpu and all is fine. > But if the count is zero you can get preempted between the instructions, > the process now running on your 'old' cpu can increment the value > and you then read the new non-zero value. > That won't be good at all. TBH, I don't think this counts for "don't care the stale value". > > You can only use __this_cpu_read() for things that don't change. > > The big problem with using per-cpu page tables is there will be absolutely > nothing stopping code taking the wrong address of a per-cpu variable and > saving it somewhere. Err... I'm lost. If you mean RW or RMW, atomic instructions are required for this_cpu ops. This is how this_cpu ops is implemented for ARM64 even though without percpu page table. If the operation is interrupted in the middle, the exclusion monitor will be cleared, the hardware will reload the value. If you mean this_cpu_read() some value then this_cpu_write() to the same cpu, I don't think it can work as expected without disabling preemption for the whole code section even though we don't have percpu page table. Thanks, Yang > At the moment you have to use the helper so always get the global address. > > -- David > >> Thanks, >> Yang