diff options
author | Steven Rostedt <rostedt@goodmis.org> | 2013-12-05 09:16:52 -0500 |
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committer | Sebastian Andrzej Siewior <bigeasy@linutronix.de> | 2016-02-13 00:36:31 +0100 |
commit | 69e64474880618bc003e1186ac5835f491cfec69 (patch) | |
tree | e446a09416ac7bb1228164270185d306343444b9 | |
parent | dd6b7f471e3463b3852805b018d30d845541aa2a (diff) | |
download | rt-linux-69e64474880618bc003e1186ac5835f491cfec69.tar.gz |
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>
-rw-r--r-- | kernel/cpu.c | 8 |
1 files changed, 8 insertions, 0 deletions
diff --git a/kernel/cpu.c b/kernel/cpu.c index c48208b921601d..34944ea666199e 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -110,6 +110,14 @@ 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; |