Subject: Re: [PATCH 1/2] smp: use '|=' for csd_lock
From: li guang <lig.fnst@cn.fujitsu.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: tglx@linutronix.de, peterz@infradead.org, shli@fusionio.com,
        srivatsa.bhat@linux.vnet.ibm.com, suresh.b.siddha@intel.com,
        fweisbec@gmail.com, sedat.dilek@gmail.com, paulmck@linux.vnet.ibm.com,
        linux-kernel@vger.kernel.org
In-Reply-To: <20130423154019.4563d819760f5c5dd07ff4f6@linux-foundation.org>
References: <1366609643-17628-1-git-send-email-lig.fnst@cn.fujitsu.com>
	 <20130423154019.4563d819760f5c5dd07ff4f6@linux-foundation.org>
Date: Wed, 24 Apr 2013 08:08:05 +0800
Message-ID: <1366762085.20507.8.camel@liguang.fnst.cn.fujitsu.com>
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在 2013-04-23二的 15:40 -0700，Andrew Morton写道：
> On Mon, 22 Apr 2013 13:47:22 +0800 liguang <lig.fnst@cn.fujitsu.com> wrote:
> 
> > originally, 'data->flags = CSD_FLAG_LOCK',
> > and we use 'data->flags &= ~CSD_FLAG_LOCK'
> > for csd_unlock, they are not symmetrix operations
> > so use '|=' instead of '='.
> > though, now data->flags only hold CSD_FLAG_LOCK,
> > it's not so meaningful to use '|=' to set 1 bit,
> > and '&= ~' to clear 1 bit.
> > 
> > Signed-off-by: liguang <lig.fnst@cn.fujitsu.com>
> > ---
> >  kernel/smp.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/kernel/smp.c b/kernel/smp.c
> > index 1818dc0..2d5deb4 100644
> > --- a/kernel/smp.c
> > +++ b/kernel/smp.c
> > @@ -109,7 +109,7 @@ static void csd_lock_wait(struct call_single_data *data)
> >  static void csd_lock(struct call_single_data *data)
> >  {
> >  	csd_lock_wait(data);
> > -	data->flags = CSD_FLAG_LOCK;
> > +	data->flags |= CSD_FLAG_LOCK;
> >  
> >  	/*
> 
> call_single_data.flags is in fact presently a boolean - we only use one
> bit in that word.  We could remove all the &=, |=, & and | operations
> on call_single_data.flags and treat it as a boolean.  That would
> probably result in faster and smaller code.
> 
> But leaving the other 31 bits alone and reserved-for-future-use is not
> a bad thing.  But if we're going to do that we should do it consistently.
> 
> I rewrote your changelog ;)

That's fine, Thanks!

> 
> 
> From: liguang <lig.fnst@cn.fujitsu.com>
> Subject: kernel/smp.c: use '|=' for csd_lock
> 
> csd_lock() uses assignment to data->flags rather than |=.  That is not
> buggy at present because only one bit (CSD_FLAG_LOCK) is defined in
> call_single_data.flags.
> 
> But it will become buggy if we later add another flag, so fix it now.
> 
> Signed-off-by: liguang <lig.fnst@cn.fujitsu.com>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Ingo Molnar <mingo@elte.hu>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  kernel/smp.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff -puN kernel/smp.c~kernel-smpc-use-=-for-csd_lock kernel/smp.c
> --- a/kernel/smp.c~kernel-smpc-use-=-for-csd_lock
> +++ a/kernel/smp.c
> @@ -110,7 +110,7 @@ static void csd_lock_wait(struct call_si
>  static void csd_lock(struct call_single_data *data)
>  {
>  	csd_lock_wait(data);
> -	data->flags = CSD_FLAG_LOCK;
> +	data->flags |= CSD_FLAG_LOCK;
>  
>  	/*
>  	 * prevent CPU from reordering the above assignment
> _
> 


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