linux-kernel - Re: [PATCH 1/3] smp/ipi: Remove redundant cfd->cpumask

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Date:	Sat, 6 Jul 2013 11:13:10 +0800
From:	Wang YanQing <udknight@...il.com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	xiaoguangrong@...fujitsu.com, mingo@...e.hu,
	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	a.p.zijlstra@...llo.nl, npiggin@...e.de,
	deepthi@...ux.vnet.ibm.com, peterz@...radead.org,
	rusty@...tcorp.com.au, heiko.carstens@...ibm.com,
	rostedt@...dmis.org, miltonm@....com,
	srivatsa.bhat@...ux.vnet.ibm.com, jens.axboe@...cle.com,
	tj@...nel.org, akpm@...ux-foundation.org,
	svaidy@...ux.vnet.ibm.com, shli@...nel.org, tglx@...utronix.de,
	lig.fnst@...fujitsu.com, anton@...ba.org,
	torvalds@...ux-foundation.org, jbeulich@...e.com
Subject: Re: [PATCH 1/3] smp/ipi: Remove redundant cfd->cpumask_ipi mask

On Fri, Jul 05, 2013 at 09:57:01PM +0530, Preeti U Murthy wrote:
> cfd->cpumask_ipi is used only in smp_call_function_many().The existing
> comment around it says that this additional mask is used because
> cfd->cpumask can get overwritten.
> 
> There is no reason why the cfd->cpumask can be overwritten, since this
> is a per_cpu mask; nobody can change it but us and we are
> called with preemption disabled.

The ChangeLog for f44310b98ddb7f0d06550d73ed67df5865e3eda5
which import cfd->cpumask_ipi saied the reason why we need
it:

"    As explained by Linus as well:
    
     |
     | Once we've done the "list_add_rcu()" to add it to the
     | queue, we can have (another) IPI to the target CPU that can
     | now see it and clear the mask.
     |
     | So by the time we get to actually send the IPI, the mask might
     | have been cleared by another IPI.
     |
    
    This patch also fixes a system hang problem, if the data->cpumask
    gets cleared after passing this point:
    
            if (WARN_ONCE(!mask, "empty IPI mask"))
                    return;
    
    then the problem in commit 83d349f35e1a ("x86: don't send an IPI to
    the empty set of CPU's") will happen again.
"
So this patch is wrong.

And you should cc linus and Jan Beulich who give acked-by tag to
the commit.

Thanks.
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