Date: Mon, 9 Sep 2013 06:48:57 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Eric Dumazet <eric.dumazet@gmail.com>, linux-kernel@vger.kernel.org,
        mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de,
        dhowells@redhat.com, edumazet@google.com, darren@dvhart.com,
        sbw@mit.edu
Subject: Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical
 section?
Message-ID: <20130909134857.GQ3966@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <20130906174117.GU3966@linux.vnet.ibm.com>
 <20130906185927.GE2706@somewhere>
 <20130909105347.GK31370@twins.programming.kicks-ass.net>
 <20130909121329.GA16280@somewhere>
 <20130909083926.3eceebef@gandalf.local.home>
 <20130909124547.GB16280@somewhere>
 <20130909085504.2ddd7e69@gandalf.local.home>
 <20130909130851.GC16280@somewhere>
 <20130909092142.05780991@gandalf.local.home>
 <20130909092917.0c99b71a@gandalf.local.home>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20130909092917.0c99b71a@gandalf.local.home>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2068
Lines: 48

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@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.

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
> 

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/