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sched/cache: Fix race condition during sched domain rebuild

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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: 067a313581 ("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
This commit is contained in:
Chen Yu 2026-05-13 13:39:23 -07:00 committed by Peter Zijlstra
parent d6b9afab44
commit 9f7c745850
3 changed files with 20 additions and 29 deletions
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4
kernel/sched/debug.c
View File

@ -224,7 +224,9 @@ sched_cache_enable_write(struct file *filp, const char __user *ubuf,
sysctl_sched_cache_user = val;
sched_cache_active_set_unlocked();
sched_cache_active_set();
*ppos += cnt;
return cnt;
}

2
kernel/sched/sched.h
View File

@ -4083,7 +4083,7 @@ static inline bool sched_cache_enabled(void)
return static_branch_unlikely(&sched_cache_active);
}
extern void sched_cache_active_set_unlocked(void);
extern void sched_cache_active_set(void);
#endif

43
kernel/sched/topology.c
View File

@ -917,30 +917,20 @@ static bool alloc_sd_llc(const struct cpumask *cpu_map,
return false;
}
static void _sched_cache_active_set(bool enable, bool locked)
{
if (enable) {
if (locked)
static_branch_enable_cpuslocked(&sched_cache_active);
else
static_branch_enable(&sched_cache_active);
} else {
if (locked)
static_branch_disable_cpuslocked(&sched_cache_active);
else
static_branch_disable(&sched_cache_active);
}
}
/*
* Enable/disable cache aware scheduling according to
* user input and the presence of hardware support.
*/
static void sched_cache_active_set(bool locked)
static void _sched_cache_active_set(void)
{
lockdep_assert_cpus_held();
lockdep_assert_held(&sched_domains_mutex);
/* hardware does not support */
if (!static_branch_likely(&sched_cache_present)) {
_sched_cache_active_set(false, locked);
static_branch_disable_cpuslocked(&sched_cache_active);
if (sched_debug())
pr_info("%s: cache aware scheduling not supported on this platform\n", __func__);
return;
}
@ -951,24 +941,23 @@ static void sched_cache_active_set(bool locked)
* for now.
*/
if (sysctl_sched_cache_user) {
_sched_cache_active_set(true, locked);
static_branch_enable_cpuslocked(&sched_cache_active);
if (sched_debug())
pr_info("%s: enabling cache aware scheduling\n", __func__);
} else {
_sched_cache_active_set(false, locked);
static_branch_disable_cpuslocked(&sched_cache_active);
if (sched_debug())
pr_info("%s: disabling cache aware scheduling\n", __func__);
}
}
static void sched_cache_active_set_locked(void)
void sched_cache_active_set(void)
{
return sched_cache_active_set(true);
}
void sched_cache_active_set_unlocked(void)
{
return sched_cache_active_set(false);
cpus_read_lock();
sched_domains_mutex_lock();
_sched_cache_active_set();
sched_domains_mutex_unlock();
cpus_read_unlock();
}
/*
@ -3082,7 +3071,7 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
else
static_branch_disable_cpuslocked(&sched_cache_present);
sched_cache_active_set_locked();
_sched_cache_active_set();
#endif
__free_domain_allocs(&d, alloc_state, cpu_map);