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 06:48:57 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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, Sep 09, 2013 at 09:29:17AM -0400, Steven Rostedt wrote:
> 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.

Ahem.  The rcu_dynticks.dynticks field really is per-CPU state: it is
tracking whether or not RCU is paying attention to the corresponding
-CPU-, not to any particular task.  When RCU wants to track tasks, it
does so with the blkd_tasks field of the rcu_node structure.

							Thanx, Paul

> 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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