linux-kernel - Re: [PATCH 04/16] rcu: Remove redundant check for an online CPU

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Date:	Wed, 23 Jul 2014 10:01:14 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"open list:READ-COPY UPDATE..." <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/16] rcu: Remove redundant check for an online CPU

On Wed, Jul 23, 2014 at 9:41 AM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Wed, Jul 23, 2014 at 09:23:47AM -0400, Pranith Kumar wrote:
>> On Wed, Jul 23, 2014 at 8:09 AM, Paul E. McKenney
>> <paulmck@...ux.vnet.ibm.com> wrote:
>> > On Wed, Jul 23, 2014 at 01:09:41AM -0400, Pranith Kumar wrote:
>> >> rcu_prcess_callbacks() is the softirq handler for RCU which is raised from
>> >> invoke_rcu_core() which is called from __call_rcu_core().
>> >>
>> >> Each of these three functions checks if the cpu is online. We can remove the
>> >> redundant ones. This commit removes one of these redundant check.
>> >>
>> >> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
>> >
>> > Sorry, but no.  There can be a long delay between raise_softirq() and
>> > this function starting, particularly if ksoftirqd gets involved.  The
>> > CPU could easily go offline in the meantime.
>> >
>>
>> That makes sense. I guess one of the other two checks in
>> __call_rcu_core() or invoke_rcu_core() can go then?
>
> What would have to be the case for this suggested change to be safe?
> Does that condition in fact hold?
>

The only scenario which is unsafe is when this thread is preempted and
scheduled on a dying CPU. In that case we should turn off interrupts.

Actually, checking the code, I see that interrupts are turned off
already before we call _call_rcu_core(). So I am not sure such a case
will happen. On the other hand invoke_rcu_core() will be a one line
function which might as well be in-lined to avoid the double cpu
online check.

What am I missing?

-- 
Pranith
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