Re: [PATCH RFC v3 net-next 3/3] samples: bpf: eBPF dropmon example in C

From: Alexei Starovoitov
Date: Wed Jul 30 2014 - 14:54:05 EST

On Wed, Jul 30, 2014 at 10:36 AM, Frank Ch. Eigler <fche@xxxxxxxxxx> wrote:
>> > What kind of locking/serialization is provided by the ebpf runtime
>> > over shared variables such as my_map?
>> it's traditional rcu scheme.
> OK, that protects the table structure, but:
>> [...] In such case concurrent write access to map value can be done
>> with bpf_xadd instruction, though using normal read/write is also
>> allowed. In some cases the speed of racy var++ is preferred over
>> 'lock xadd'.
> ... so concurrency control over shared values is left up to the
> programmer.

yes. It has to be flexible and fast.
One of our main use cases is network analytics where a lot of
packets are going through ebpf programs, so every cycle counts.
Mandatory locks in critical path are not acceptable. If we add
locks they will be optional.

>> There are no lock/unlock function helpers available to ebpf
>> programs, since program may terminate early with div by zero
>> for example, so in-kernel lock helper implementation would
>> be complicated and slow. It's possible to do, but for the use
>> cases so far there is no need.
> OK, I hope that works out. I've been told that dtrace does something
> similiar (!) by eschewing protection on global variables such as
> strings. In their case it's less bad than it sounds because they are
> used to offloading computation to userspace or to store only
> thread-local state, and accept the corollary limitations on control.

btw, things like global variables, per-cpu storage are potential ebpf
features. So far they're 'nice to have' instead of 'mandatory'.
The maps are powerful enough to do the same:
Global storage is map of one element.
Per-cpu storage is map of num_cpu elements.
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