On 12/13/2012 09:47 PM, Oleg Nesterov wrote: > On 12/13, Srivatsa S. Bhat wrote: >> >> On 12/13/2012 12:42 AM, Srivatsa S. Bhat wrote: >>> >>> Even I don't spot anything wrong with it. But I'll give it some more >>> thought.. >> >> Since an interrupt handler can also run get_online_cpus_atomic(), we >> cannot use the __this_cpu_* versions for modifying reader_percpu_refcnt, >> right? > > Hmm. I thought that __this_cpu_* must be safe under preempt_disable(). > IOW, I thought that, say, this_cpu_inc() is "equal" to preempt_disable + > __this_cpu_inc() correctness-wise. > > And. I thought that this_cpu_inc() is safe wrt interrupt, like local_t. > > But when I try to read the comments percpu.h, I am starting to think that > even this_cpu_inc() is not safe if irq handler can do the same? >
The comment seems to say that its not safe wrt interrupts. But looking at the code in include/linux/percpu.h, IIUC, that is true only about this_cpu_read() because it only disables preemption. However, this_cpu_inc() looks safe wrt interrupts because it wraps the increment within raw_local_irqsave()/restore(). > Confused... > > I am shy to ask... will, say, DEFINE_PER_CPU(local_t) and > local_inc(__this_cpu_ptr(...)) work?? > >> But still, this scheme is better, because the reader doesn't have to spin >> on the read_lock() with interrupts disabled. > > Yes, but my main concern is that irq_disable/enable itself is not that cheap. > Regards, Srivatsa S. Bhat -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
