linux-kernel - Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical section?

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Date:	Mon, 9 Sep 2013 09:29:17 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, sbw@....edu
Subject: Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical
 section?

On Mon, 9 Sep 2013 09:21:42 -0400
Steven Rostedt <rostedt@...dmis.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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