Commit 0ec8d5ae authored by Peilin He's avatar Peilin He Committed by Peter Zijlstra

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: default avatarPeter Zijlstra <peterz@infradead.org>
Signed-off-by: Peilin He<he.peilin@zte.com.cn>
Signed-off-by: default avatarxu xin <xu.xin16@zte.com.cn>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarYunkai Zhang <zhang.yunkai@zte.com.cn>
Reviewed-by: default avatarQiang Tu <tu.qiang35@zte.com.cn>
Reviewed-by: default avatarKun Jiang <jiang.kun2@zte.com.cn>
Reviewed-by: default avatarFan Yu <fan.yu9@zte.com.cn>
Link: https://lkml.kernel.org/r/20240716104244764N2jD8gnBpnsLjCDnQGQ8c@zte.com.cn
parent c40dd90a
...@@ -2233,6 +2233,12 @@ void migrate_disable(void) ...@@ -2233,6 +2233,12 @@ void migrate_disable(void)
struct task_struct *p = current; struct task_struct *p = current;
if (p->migration_disabled) { 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++; p->migration_disabled++;
return; return;
} }
...@@ -2251,14 +2257,20 @@ void migrate_enable(void) ...@@ -2251,14 +2257,20 @@ void migrate_enable(void)
.flags = SCA_MIGRATE_ENABLE, .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) { if (p->migration_disabled > 1) {
p->migration_disabled--; p->migration_disabled--;
return; return;
} }
if (WARN_ON_ONCE(!p->migration_disabled))
return;
/* /*
* Ensure stop_task runs either before or after this, and that * Ensure stop_task runs either before or after this, and that
* __set_cpus_allowed_ptr(SCA_MIGRATE_ENABLE) doesn't schedule(). * __set_cpus_allowed_ptr(SCA_MIGRATE_ENABLE) doesn't schedule().
......
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