Hello, Mark. On Mon, Sep 25, 2017 at 02:24:32PM +0100, Mark Rutland wrote: > As raw_cpu_generic_read() is a plain read from a raw_cpu_ptr() address, > it's possible (albeit unlikely) that the compiler will split the access > across multiple instructions. > > In this_cpu_generic_read() we disable preemption but not interrupts > before calling raw_cpu_generic_read(). Thus, an interrupt could be taken > in the middle of the split load instructions. If a this_cpu_write() or > RMW this_cpu_*() op is made to the same variable in the interrupt > handling path, this_cpu_read() will return a torn value. > > Avoid this by using READ_ONCE() to inhibit tearing. That's why there are irq-safe variants of the operations. Adding READ_ONCE() doesn't generically guarantee that the reads won't be split - e.g. there are arch which simply can't load a 64bit value with a single instruction. Thanks. -- tejun
