sched/cache: Fix race condition during sched domain rebuild

sched_cache_active_set_unlocked() checks hardware support without
locks:
static void sched_cache_active_set(bool locked)
{
        /* hardware does not support */
        if (!static_branch_likely(&sched_cache_present)) {
                _sched_cache_active_set(false, locked);
                return;
        }
    ...
If build_sched_domains() runs concurrently during CPU hotplug,
it can disable sched_cache_present under sched_domains_mutex
and the CPU hotplug lock. If a debugfs write thread evaluates
sched_cache_present as true right before that, and then blocks
or gets preempted, it might proceed to enable sched_cache_active
after the hardware support has been marked as absent. Make it
safer by acquiring cpus_read_lock() and sched_domains_mutex_lock()
when the user changes sched_cache_active via debugfs.

This bug was reported by sashiko.

Fixes: 067a31358143 ("sched/cache: Allow the user space to turn on and off cache aware scheduling")
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Co-developed-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/9afddf439687f04bb56b46625bd9f153eb8abad5.1778703694.git.tim.c.chen@linux.intel.com

0 files changed, 0 insertions, 0 deletions