On Mon, Sep 10, 2018 at 01:32:15 +0200, Paolo Bonzini wrote: > On 03/09/2018 19:18, Emilio G. Cota wrote: > > Using atomics here is a mistake since they're not guaranteed > > to compile. > > But isn't it technically a C11 data race if you don't use atomics?
Yes, it's undefined behaviour. > Could we make nocheck read/set degrade to just a volatile access when > used on a variable that is bigger than pointers, or perhaps always > except when using tsan? But volatile wouldn't save you from undefined behaviour, would it? A simpler and definitely correct alternative is to just use a spinlock instead of the seqlock also for reads when !CONFIG_ATOMIC64. We don't care about scalability on those rare hosts anyway, so I'd go with that. Thanks, Emilio