On 8/11/23 4:36 PM, David Vernet wrote:
I see, thanks for explaining. This is why sched_ext doesn't really work
with the BPF_F_LINK version of map update. We can't guarantee that a map
can be updated if we can't succeed in ->reg(), because we can also race
with e.g. sysrq unloading the scheduler between ->validate() and
->reg(). In a sense, it feels like ->reg() in "updateable" struct_ops
implementations should be void, whereas in other struct_ops
implementations like scx() it has to be int *. If validate() is meant to
prevent the scenario you outlined, can you help me understand why we
still check the return value of ->reg() in bpf_struct_ops_link_create()?
Or at the very least it seems like we should WARN_ON()?
->regs() can fail if another struct_ops under the same name has already been
loaded to the subsystem. If another subsystem needs another return value to
support .update, I believe it can be done if that is blocking scx to support
"updateable" link.
If it needs to validate struct_ops as a while,
There was a typo: as a /whole/.
1. it must be implemented in .validate instead of .reg. Otherwise, it may
end up having an unusable map.
Some clarity on this point (why we check ->reg() on the ->validate()
path) would help me write this comment more clearly.
hmm... where does it check ->reg() on the ->validate() now?
I was meaning the struct_ops supported subsystem should validate the struct_ops
map in '.validate' instead of in the '.reg'.
or I misunderstood the question?
2. if the validation is implemented in '.reg' only, the map update behavior
will be different between BPF_F_LINK map and the non BPF_F_LINK map.
Ack, this is good to document regardless.
I'll send a v3 on Monday with these comments added both to the code, and
to the commit summary.
Thanks,
David
