Jetpack/kernel/kernel-4.9/rt-patches/0282-cpu-hotplug-Document-w...

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From 4150ad7e6dd6aea34d82120502be90e5fe4073cd Mon Sep 17 00:00:00 2001
From: Steven Rostedt <rostedt@goodmis.org>
Date: Thu, 5 Dec 2013 09:16:52 -0500
Subject: [PATCH 282/352] cpu hotplug: Document why PREEMPT_RT uses a spinlock
The patch:
cpu: Make hotplug.lock a "sleeping" spinlock on RT
Tasks can block on hotplug.lock in pin_current_cpu(), but their
state might be != RUNNING. So the mutex wakeup will set the state
unconditionally to RUNNING. That might cause spurious unexpected
wakeups. We could provide a state preserving mutex_lock() function,
but this is semantically backwards. So instead we convert the
hotplug.lock() to a spinlock for RT, which has the state preserving
semantics already.
Fixed a bug where the hotplug lock on PREEMPT_RT can be called after a
task set its state to TASK_UNINTERRUPTIBLE and before it called
schedule. If the hotplug_lock used a mutex, and there was contention,
the current task's state would be turned to TASK_RUNNABLE and the
schedule call will not sleep. This caused unexpected results.
Although the patch had a description of the change, the code had no
comments about it. This causes confusion to those that review the code,
and as PREEMPT_RT is held in a quilt queue and not git, it's not as easy
to see why a change was made. Even if it was in git, the code should
still have a comment for something as subtle as this.
Document the rational for using a spinlock on PREEMPT_RT in the hotplug
lock code.
Reported-by: Nicholas Mc Guire <der.herr@hofr.at>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
kernel/cpu.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 87424f1..583f6ac 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -267,6 +267,14 @@ struct hotplug_pcp {
int grab_lock;
struct completion synced;
#ifdef CONFIG_PREEMPT_RT_FULL
+ /*
+ * Note, on PREEMPT_RT, the hotplug lock must save the state of
+ * the task, otherwise the mutex will cause the task to fail
+ * to sleep when required. (Because it's called from migrate_disable())
+ *
+ * The spinlock_t on PREEMPT_RT is a mutex that saves the task's
+ * state.
+ */
spinlock_t lock;
#else
struct mutex mutex;
--
2.7.4