Re: [PATCH V5 1/1] bpf: control events stored in PERF_EVENT_ARRAY maps trace data output when perf sampling

On Wed, Oct 21, 2015 at 07:49:34PM +0800, Wangnan (F) wrote:

If our task is sampling cycle events during a function is running,
and if two cores start that function overlap:

Time:   ...................A
Core 0: sys_write----\
                      \
                       \
Core 1:             sys_write%return
Core 2: ................sys_write

Then without counter at time A it is highly possible that
BPF program on core 1 and core 2 get conflict with each other.
The final result is we make some of those events be turned on
and others turned off. Using atomic counter can avoid this
problem.

But but, how and why can an eBPF program access a !local event? I
thought we had hard restrictions on that.