Re: [PATCH tip 0/9] tracing: attach eBPF programs to tracepoints/syscalls/kprobe

On Wed, Jan 21, 2015 at 5:56 PM, Steven Rostedt [off-list ref] wrote:

On Wed, 21 Jan 2015 17:49:08 -0800
Alexei Starovoitov [off-list ref] wrote:

quoted

quoted

This also makes it keeping events in the soft-disabled state.

I was never able to figure out the use case for soft-disabled state.
Probably historical before static_key was done.

No, it's not historical at all. The "soft-disable" is a way to enable
from any context. You can't enable a static key from NMI or interrupt
context, but you can enable a "soft-disable" there.

As you can enable or disable events from any function that the function
tracer may trace, I needed a way to enable them (make the tracepoint
active), but do nothing until something else turns them on.

Thanks for explanation. Makes sense.
Speaking of nmi... I think I will add a check that if (in_nmi())
just skip running the program, since supporting this use
case is not needed at the moment.