Re: [PATCH bpf-next v1 0/3] Update KF_RCU_PROTECTED, add KF_RET_RCU

[Andrea Righi <arighi@nvidia.com>]

On Mon, Sep 15, 2025 at 02:47:28AM +0000, Kumar Kartikeya Dwivedi wrote:
> Currently, KF_RCU_PROTECTED only applies to iterator APIs and that too
> in a convoluted fashion: the presence of this flag on the kfunc is used
> to set MEM_RCU in iterator type, and the lack of RCU protection results
> in an error only later, once next() or destroy() methods are invoked on
> the iterator. While there is no bug, this is certainly a bit unintuitive,
> and makes the enforcement of the flag iterator specific.
> 
> In the interest of making this flag useful for other upcoming kfuncs,
> e.g. scx_bpf_cpu_curr() [0][1], add enforcement for invoking the kfunc
> in an RCU critical section in general.
> 
> In addition to this, the aforementioned kfunc also needs to return an
> RCU protected pointer, which currently has no generic kfunc flag or
> annotation. Add such a flag as well while we are at it.
> 
>   [0]: https://lore.kernel.org/all/20250903212311.369697-3-christian.loehle@arm.com
>   [1]: https://lore.kernel.org/all/20250909195709.92669-1-arighi@nvidia.com

In the meantime, everything looks good from a sched_ext perspective. With
this applied we can implement the correct scx_bpf_cpu_curr() behavior.

Tested-by: Andrea Righi <arighi@nvidia.com>

Thanks,
-Andrea