5.4-stable review patch. If anyone has any objections, please let me know.
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From: Koichiro Den koichiro.den@canonical.com
commit 2f8dea1692eef2b7ba6a256246ed82c365fdc686 upstream.
Consider a scenario where a CPU transitions from CPUHP_ONLINE to halfway through a CPU hotunplug down to CPUHP_HRTIMERS_PREPARE, and then back to CPUHP_ONLINE:
Since hrtimers_prepare_cpu() does not run, cpu_base.hres_active remains set to 1 throughout. However, during a CPU unplug operation, the tick and the clockevents are shut down at CPUHP_AP_TICK_DYING. On return to the online state, for instance CFS incorrectly assumes that the hrtick is already active, and the chance of the clockevent device to transition to oneshot mode is also lost forever for the CPU, unless it goes back to a lower state than CPUHP_HRTIMERS_PREPARE once.
This round-trip reveals another issue; cpu_base.online is not set to 1 after the transition, which appears as a WARN_ON_ONCE in enqueue_hrtimer().
Aside of that, the bulk of the per CPU state is not reset either, which means there are dangling pointers in the worst case.
Address this by adding a corresponding startup() callback, which resets the stale per CPU state and sets the online flag.
[ tglx: Make the new callback unconditionally available, remove the online modification in the prepare() callback and clear the remaining state in the starting callback instead of the prepare callback ]
Fixes: 5c0930ccaad5 ("hrtimers: Push pending hrtimers away from outgoing CPU earlier") Signed-off-by: Koichiro Den koichiro.den@canonical.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Cc: stable@vger.kernel.org Link: https://lore.kernel.org/all/20241220134421.3809834-1-koichiro.den@canonical.... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- include/linux/hrtimer.h | 1 + kernel/cpu.c | 2 +- kernel/time/hrtimer.c | 11 ++++++++++- 3 files changed, 12 insertions(+), 2 deletions(-)
--- a/include/linux/hrtimer.h +++ b/include/linux/hrtimer.h @@ -527,6 +527,7 @@ extern void __init hrtimers_init(void); extern void sysrq_timer_list_show(void);
int hrtimers_prepare_cpu(unsigned int cpu); +int hrtimers_cpu_starting(unsigned int cpu); #ifdef CONFIG_HOTPLUG_CPU int hrtimers_cpu_dying(unsigned int cpu); #else --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -1542,7 +1542,7 @@ static struct cpuhp_step cpuhp_hp_states }, [CPUHP_AP_HRTIMERS_DYING] = { .name = "hrtimers:dying", - .startup.single = NULL, + .startup.single = hrtimers_cpu_starting, .teardown.single = hrtimers_cpu_dying, },
--- a/kernel/time/hrtimer.c +++ b/kernel/time/hrtimer.c @@ -2065,6 +2065,15 @@ int hrtimers_prepare_cpu(unsigned int cp }
cpu_base->cpu = cpu; + hrtimer_cpu_base_init_expiry_lock(cpu_base); + return 0; +} + +int hrtimers_cpu_starting(unsigned int cpu) +{ + struct hrtimer_cpu_base *cpu_base = this_cpu_ptr(&hrtimer_bases); + + /* Clear out any left over state from a CPU down operation */ cpu_base->active_bases = 0; cpu_base->hres_active = 0; cpu_base->hang_detected = 0; @@ -2073,7 +2082,6 @@ int hrtimers_prepare_cpu(unsigned int cp cpu_base->expires_next = KTIME_MAX; cpu_base->softirq_expires_next = KTIME_MAX; cpu_base->online = 1; - hrtimer_cpu_base_init_expiry_lock(cpu_base); return 0; }
@@ -2151,6 +2159,7 @@ int hrtimers_cpu_dying(unsigned int dyin void __init hrtimers_init(void) { hrtimers_prepare_cpu(smp_processor_id()); + hrtimers_cpu_starting(smp_processor_id()); open_softirq(HRTIMER_SOFTIRQ, hrtimer_run_softirq); }