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

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Date:	Tue, 12 Jan 2016 21:30:09 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Ming Lei <tom.leiming@...il.com>
Cc:	Martin KaFai Lau <kafai@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alexei Starovoitov <ast@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Network Development <netdev@...r.kernel.org>,
	Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH 5/9] bpf: syscall: add percpu version of lookup/update
 elem

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@...com> 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.