Re: [PATCH 5/9] bpf: syscall: add percpu version of lookup/update elem

From: Alexei Starovoitov
Date: Wed Jan 13 2016 - 00:29:52 EST

On Wed, Jan 13, 2016 at 11:17:23AM +0800, Ming Lei wrote:
> On Wed, Jan 13, 2016 at 10:22 AM, Martin KaFai Lau <kafai@xxxxxx> wrote:
> > On Wed, Jan 13, 2016 at 08:38:18AM +0800, Ming Lei wrote:
> >> > The userspace usually only aggregates value across all cpu every X seconds.
> >>
> >> That is just in your case, and Alexei worried the issue of data stale.
> > I believe we are talking about validity of a value. How to
> > make use of a less-stale but invalid data?
> About the 'invalidity' thing, it should be same between using
> smp_call(run in IPI irq handler) and simple memcpy().
> When smp_call_function_single() is used to request to lookup element in
> the specific CPU, the value of the element may be in updating in that CPU
> and not completed yet in eBPF prog, then IPI comes and half updated
> data is still returned to syscall.

hmm. I'm not following. bpf programs are executing with preempt disabled,
so smp_call_function_single suppose to execute when bpf is not running.