linux-kernel - Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE

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Date:	Wed, 23 Feb 2011 13:17:07 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE_RCU from softirq
 to kthread

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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