Re: [PATCH v2 5/5] sched/fair: cleanup for SIS_PROP

[Mel Gorman]

On Thu, Sep 01, 2022 at 09:11:07PM +0800, Abel Wu wrote:
> The sched-domain of this cpu is only used when SIS_PROP is enabled,
> and it should be irrelevant whether the local sd_llc is valid or
> not, since all we care about is target sd_llc if !SIS_PROP.
> 
> Signed-off-by: Abel Wu <wuyun.abel@xxxxxxxxxxxxx>

This could conceivably result in an uninitialised memory access if
SIS_PROP was enabled while select_idle_cpu is running. I'm not sure if
it can happen when jump labels are in use but I think it could happen
for !CONFIG_JUMP_LABEL updating the sysctl_sched_features bitmap updated
via sysctl.

The patch is still a good idea because it moves an unlikely rcu_deference
out of the default path for sched features but either this_sd needs to
be initialised to NULL and checked or the this_sd lookup needs to happen
twice at a slight additional cost to the default-disabled SIS_PROP path.

-- 
Mel Gorman
SUSE Labs