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Re: CPU Concurrency Issues

Matt Ahrens
 

On Mon, Aug 19, 2019 at 4:30 PM Yonghong Song <ys114321@...> wrote:
On Mon, Aug 19, 2019 at 1:54 PM <brad.lewis@...> wrote:
>
> Hi all,
>
> I'm trying to verify that there are no concurrency issues with an approach I'm using cpu_id as a key to a HASH_MAP.   My understanding is that bcc disables preemption but the details are fuzzy and I haven't been able to find anything in the code.  Can anyone shed some light on this?

preemption is a kernel thing, bcc does not disable it.  You need to
check kernel configuration CONFIG_PREEMPT in your host.

When running bpf code from a kprobe / kretprobe, does anything ensure that cpu_id doesn't change while the bpf is running (e.g. due to preemption)?  Does anything ensure that no other bpf code runs on this CPU while this kprobe is running (e.g. due to an interrupt firing and hitting a different kprobe)?  If either of those things can happen, it seems difficult to atomically increment an entry in a HASH_MAP (even when using the cpu_id as a key).

--matt
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