linux-kernel - Re: [PATCH 3/3] smp/ipi:Remove check around csd lock in handler for smp_call

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Message-ID: <20130706054552.GA2929@udknight>
Date:	Sat, 6 Jul 2013 13:45:53 +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
Subject: Re: [PATCH 3/3] smp/ipi:Remove check around csd lock in handler for
 smp_call_function variants

On Fri, Jul 05, 2013 at 09:57:21PM +0530, Preeti U Murthy wrote:
> call_single_data is always locked by all callers of
> arch_send_call_function_single_ipi() or
> arch_send_call_function_ipi_mask() which results in execution of
> generic_call_function_interrupt() handler.
> 
> Hence remove the check for lock on csd in generic_call_function_interrupt()
> handler, before unlocking it.

I can't find where is the generic_call_function_interrupt :)

> Signed-off-by: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: Xiao Guangrong <xiaoguangrong@...fujitsu.com>
> Cc: srivatsa.bhat@...ux.vnet.ibm.com
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Rusty Russell <rusty@...tcorp.com.au
> ---
> 
>  kernel/smp.c |   14 +-------------
>  1 file changed, 1 insertion(+), 13 deletions(-)
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index b6981ae..d37581a 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -181,25 +181,13 @@ void generic_smp_call_function_single_interrupt(void)
>  
>  	while (!list_empty(&list)) {
>  		struct call_single_data *csd;
> -		unsigned int csd_flags;
>  
>  		csd = list_entry(list.next, struct call_single_data, list);
>  		list_del(&csd->list);
>  
> -		/*
> -		 * 'csd' can be invalid after this call if flags == 0
> -		 * (when called through generic_exec_single()),
> -		 * so save them away before making the call:
> -		 */
> -		csd_flags = csd->flags;
> -

You haven't mention this change in the ChangeLog, don't do it.
I can't see any harm to remove csd_flags, but I hope others
check it again.

>  		csd->func(csd->info);
>  
> -		/*
> -		 * Unlocked CSDs are valid through generic_exec_single():
> -		 */
> -		if (csd_flags & CSD_FLAG_LOCK)
> -			csd_unlock(csd);
> +		csd_unlock(csd);

I don't like this change, I think check CSD_FLAG_LOCK 
to make sure we really need csd_unlock is good.

Just like you can't know who and how people will use the
API, so some robust check code is good.
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