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Date: Mon, 9 Sep 2013 08:39:26 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        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?
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On Mon, 9 Sep 2013 14:13:31 +0200
Frederic Weisbecker <fweisbec@gmail.com> wrote:


> > In any case the preempt_disable/enable pair there is just plain wrong as
> > Eric pointed out.
> 
> Check this:
> 
> 34240697d619c439c55f21989680024dcb604aab "rcu: Disable preemption in rcu_is_cpu_idle()"


Ug, and that patch does nothing to fix the bug that it reported!

    1.  Task A on CPU 1 enters rcu_is_cpu_idle() and picks up the
    pointer to CPU 1's per-CPU variables.
    
    2.  Task B preempts Task A and starts running on CPU 1.

Let's say that B preempts Task A here:

       preempt_disable();
       ret = (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) &  0x1) == 0;
       preempt_enable();
 <preempt>
       return ret;

    
    3.  Task A migrates to CPU 2.
    
    4.  Task B blocks, leaving CPU 1 idle.
    
    5.  Task A continues execution on CPU 2, accessing CPU 1's
    dyntick-idle information using the pointer fetched in step 1 above,
    and finds that CPU 1 is idle.

Yeah, and Task A is using the "ret" from CPU 1!
    
    6.  Task A therefore incorrectly concludes that it is executing in
        an extended quiescent state, possibly issuing a spurious splat.
    
    Therefore, this commit disables preemption within the
    rcu_is_cpu_idle() function.

Where this commit is totally bogus. Sorry, but it is.

This just proves that the caller of rcu_is_cpu_idle() must disable
preemption itself for the entire time that it needs to use the result
of rcu_is_cpu_idle().

-- Steve
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