On 20.06.2017 22:37, Tejun Heo wrote: > Hello, Nikolay. > > On Tue, Jun 20, 2017 at 09:02:00PM +0300, Nikolay Borisov wrote: >> Currently the writeback statistics code uses a percpu counters to hold >> various statistics. Furthermore we have 2 families of functions - those which >> disable local irq and those which doesn't and whose names begin with >> double underscore. However, they both end up calling __add_wb_stats which in >> turn calls percpu_counter_add_batch which is already irq-safe. > > Heh, looks like I was confused. __percpu_counter_add() is not > irq-safe. It disables preemption and uses __this_cpu_read(), so > there's no protection against irq. If writeback statistics want > irq-safe operations and it does, it would need these separate > operations. Am I missing something?
So looking at the history of the commit initially there was preempt_disable + this_cpu_ptr which was later changed in: 819a72af8d66 ("percpucounter: Optimize __percpu_counter_add a bit through the use of this_cpu() options.") I believe that having __this_cpu_read ensures that we get an atomic snapshot of the variable but when we are doing the actual write e.g. the else {} branch we actually use this_cpu_add which ought to be preempt + irq safe, meaning we won't get torn write. In essence we have atomic reads by merit of __this_cpu_read + atomic writes by merit of using raw_spin_lock_irqsave in the if() branch and this_cpu_add in the else {} branch. > > Thanks. >