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

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Date:	Wed, 23 Feb 2011 12:29:59 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Steven Rostedt <rostedt@...dmis.org>
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, 23 Feb 2011, Steven Rostedt wrote:

> 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.

Right if the kthread context can be rescheduled then either preemption
needs to be disabled to guarantee that all three access the same per cpu
area data or the code needs to be changed in such a way that a this_cpu
RMW instructions can do the mods in one go.

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