From 0ec8d5aed4d14055aab4e2746def33f8b0d409c3 Mon Sep 17 00:00:00 2001 From: Peilin He Date: Tue, 16 Jul 2024 10:42:44 +0800 Subject: [PATCH] sched/core: Add WARN_ON_ONCE() to check overflow for migrate_disable() Background ========== When repeated migrate_disable() calls are made with missing the corresponding migrate_enable() calls, there is a risk of 'migration_disabled' going upper overflow because 'migration_disabled' is a type of unsigned short whose max value is 65535. In PREEMPT_RT kernel, if 'migration_disabled' goes upper overflow, it may make the migrate_disable() ineffective within local_lock_irqsave(). This is because, during the scheduling procedure, the value of 'migration_disabled' will be checked, which can trigger CPU migration. Consequently, the count of 'rcu_read_lock_nesting' may leak due to local_lock_irqsave() and local_unlock_irqrestore() occurring on different CPUs. Usecase ======== For example, When I developed a driver, I encountered a warning like "WARNING: CPU: 4 PID: 260 at kernel/rcu/tree_plugin.h:315 rcu_note_context_switch+0xa8/0x4e8" warning. It took me half a month to locate this issue. Ultimately, I discovered that the lack of upper overflow detection mechanism in migrate_disable() was the root cause, leading to a significant amount of time spent on problem localization. If the upper overflow detection mechanism was added to migrate_disable(), the root cause could be very quickly and easily identified. Effect ====== Using WARN_ON_ONCE() to check if 'migration_disabled' is upper overflow can help developers identify the issue quickly. Suggested-by: Peter Zijlstra Signed-off-by: Peilin He Signed-off-by: xu xin Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Yunkai Zhang Reviewed-by: Qiang Tu Reviewed-by: Kun Jiang Reviewed-by: Fan Yu Link: https://lkml.kernel.org/r/20240716104244764N2jD8gnBpnsLjCDnQGQ8c@zte.com.cn --- kernel/sched/core.c | 18 +++++++++++++++--- 1 file changed, 15 insertions(+), 3 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 2c61b4fcb15e..db5823fcc31c 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2233,6 +2233,12 @@ void migrate_disable(void) struct task_struct *p = current; if (p->migration_disabled) { +#ifdef CONFIG_DEBUG_PREEMPT + /* + *Warn about overflow half-way through the range. + */ + WARN_ON_ONCE((s16)p->migration_disabled < 0); +#endif p->migration_disabled++; return; } @@ -2251,14 +2257,20 @@ void migrate_enable(void) .flags = SCA_MIGRATE_ENABLE, }; +#ifdef CONFIG_DEBUG_PREEMPT + /* + * Check both overflow from migrate_disable() and superfluous + * migrate_enable(). + */ + if (WARN_ON_ONCE((s16)p->migration_disabled <= 0)) + return; +#endif + if (p->migration_disabled > 1) { p->migration_disabled--; return; } - if (WARN_ON_ONCE(!p->migration_disabled)) - return; - /* * Ensure stop_task runs either before or after this, and that * __set_cpus_allowed_ptr(SCA_MIGRATE_ENABLE) doesn't schedule(). -- 2.50.1