| From: Steven Rostedt <rostedt@goodmis.org> |
| Date: Thu, 5 Dec 2013 09:16:52 -0500 |
| Subject: 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(+) |
| |
| --- a/kernel/cpu.c |
| +++ b/kernel/cpu.c |
| @@ -260,6 +260,14 @@ 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; |