Subject: Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE_RCU from softirq
 to kthread
From: Steven Rostedt <rostedt@goodmis.org>
To: Christoph Lameter <cl@linux.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org, josh@joshtriplett.org,
        niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org,
        Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com,
        darren@dvhart.com, "Paul E. McKenney" <paul.mckenney@linaro.org>
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Date: Wed, 23 Feb 2011 13:17:07 -0500
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On Wed, 2011-02-23 at 11:34 -0600, Christoph Lameter wrote:

> > > True, but we could also argue that the multiple checks for being preempt
> > > can also be an issue.
> >
> > At least on x86 preemption don't actually need to be disabled: selection
> > of the right per-cpu memory location is done atomically with the rest of
> > the instruction by the segment selector.
> 
> Right.

But a test still needs to be made. Because three access of this_cpu_*()
that gets preempted and scheduled on another CPU can access a different
CPU var for each access. This does not matter how atomic the
this_cpu_*() code is.

IOW, the use of this_cpu_*() without preemption disabled is 99% of the
time a bug.

-- Steve


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