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
0ec8d5aed4