Messages in this thread | | | Date | Mon, 9 Sep 2013 09:29:17 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical section? |
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On Mon, 9 Sep 2013 09:21:42 -0400 Steven Rostedt <rostedt@goodmis.org> wrote:
> Especially since the function name itself is "rcu_is_cpu_idle()" which > tells you it's a cpu state, and not a task state. Why would you care in > RCU if CPU 1 is idle if you are on CPU 2? The name should be changed.
> Actually, preempt_count is more a CPU state than a task state. When > preempt_count is set, that CPU can not schedule out the current task. > It really has nothing to do with the task itself. It's the CPU that > can't do something. Preempt count should never traverse with a task > from one CPU to another.
I'll take this a step further. Here's a simple rule to determine if something is a task state or a CPU state.
If the state migrates with a task from one CPU to another, it's a task state.
If the state never leaves a CPU with a task, then it's a CPU state.
According to the above rules, rcu_is_cpu_idle() is a task state, and really should be in task_struct, and preempt_count is a CPU state, and should be a per_cpu variable.
I think the reason preempt_count never was a per cpu variable, is that having it in the stack (thread info) made it easier to test in assembly than having to grab the per cpu info. But I believe it's easier to grab per cpu info in assembly today than it once was, which is why there is a push to move preempt_count to per_cpu where it truly belongs.
-- Steve
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