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 <peterz@infradead.org>
Signed-off-by: Peilin He<he.peilin@zte.com.cn>
Signed-off-by: xu xin <xu.xin16@zte.com.cn>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Yunkai Zhang <zhang.yunkai@zte.com.cn>
Reviewed-by: Qiang Tu <tu.qiang35@zte.com.cn>
Reviewed-by: Kun Jiang <jiang.kun2@zte.com.cn>
Reviewed-by: Fan Yu <fan.yu9@zte.com.cn>
Link: https://lkml.kernel.org/r/20240716104244764N2jD8gnBpnsLjCDnQGQ8c@zte.com.cn

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2c61b4fcb15e9d23e4fe8d238900c1efa358ea30..db5823fcc31c6dac0fa4a154aa1a8022a94ce1e4 100644 (file)
--- 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().