Re: [PATCH v2] cgroup: Add preemption protection to css_rstat_updated()

[Waiman Long <llong@redhat.com>]

On 11/20/25 11:06 PM, Jiayuan Chen wrote:
> BPF programs do not disable preemption, they only disable migration.
> Therefore, when running the cgroup_hierarchical_stats selftest, a
> warning [1] is generated.
>
> The css_rstat_updated() function is lockless and reentrant. However,
> as Tejun pointed out [2], preemption-related considerations need to
> be considered. Since css_rstat_updated() can be called from BPF where
> preemption is not disabled by its framework and it has already been
> exposed as a kfunc to BPF programs, introducing a new kfunc like bpf_xx
> will break existing uses. Thus, we directly make css_rstat_updated()
> preempt-safe here.

My understand of Tejun's comment is to add bpf_preempt_disable() and 
bpf_preempt_enable() calls around the css_rstat_updated() call in the 
bpf program defined in 
tools/testing/selftests/bpf/prog_tests/cgroup_hierarchical_stats.c 
instead of adding that in the css_rstat_updated() function itself. But I 
may be wrong.

Cheers, Longman