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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>
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