linux-kernel - Re: [PATCH] smp: kill unnecessary variable

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Message-ID: <20130729121305.GE3008@twins.programming.kicks-ass.net>
Date:	Mon, 29 Jul 2013 14:13:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Xie XiuQi <xiexiuqi@...wei.com>
Cc:	Jens Axboe <jens.axboe@...cle.com>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hanjun Guo <guohanjun@...wei.com>,
	Li Zefan <lizefan@...wei.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] smp: kill unnecessary variable - csd_flags

On Mon, Jul 29, 2013 at 10:29:45AM +0800, Xie XiuQi wrote:
> We used csd_flags formerly because we allocated csd_data by
> kmalloc when "wait == 0". When fail to allocation, we will
> fall back to on-stack allocation. "csd_data" might be invalid
> after generic_exec_single return.
> 
> But now we use per cpu data for single cpu ipi calls, and
> csd_data can't fall back to on-stack allocation when "wait == 0".
> 
> So csd_flags is unnecessary now. Remove it.

The much simpler argument is that both callsites of
generic_exec_single() do an unconditional csd_lock().

> Signed-off-by: Xie XiuQi <xiexiuqi@...wei.com>
> ---
>  kernel/smp.c |   11 +----------
>  1 files changed, 1 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index 4dba0f7..cac2b6e 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -186,25 +186,16 @@ 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;
> -
>  		csd->func(csd->info);
> 
>  		/*
>  		 * Unlocked CSDs are valid through generic_exec_single():
>  		 */
> -		if (csd_flags & CSD_FLAG_LOCK)
> -			csd_unlock(csd);
> +		csd_unlock(csd);

The comment is completely useless and confusing after this; why do you
leave it in? 
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