Message-ID: <43C2B624.C70A4D6E@tv-sign.ru>
Date: Mon, 09 Jan 2006 22:14:44 +0300
From: Oleg Nesterov <oleg@tv-sign.ru>
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To: paulmck@us.ibm.com
Cc: linux-kernel@vger.kernel.org, Dipankar Sarma <dipankar@in.ibm.com>,
       Manfred Spraul <manfred@colorfullife.com>,
       Linus Torvalds <torvalds@osdl.org>, Andrew Morton <akpm@osdl.org>,
       vatsa@in.ibm.com
Subject: Re: [PATCH 5/5][RFC] rcu: start new grace period from rcu_pending()
References: <43C165D7.6EAB8E47@tv-sign.ru> <43C27417.AA1BA306@tv-sign.ru> <20060109173656.GC14738@us.ibm.com>
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Sender: linux-kernel-owner@vger.kernel.org
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"Paul E. McKenney" wrote:
> 
> On Mon, Jan 09, 2006 at 05:32:55PM +0300, Oleg Nesterov wrote:
> > Oleg Nesterov wrote:
> > >
> > > I think it is better to set ->qs_pending = 1 directly in __rcu_pending():
> >
> > This patch has a bug. I am sending a trivial fix, but now I am not
> > sure myself that 1 timer tick saved worth the code uglification.
> 
> This is indeed an accident waiting to happen -- someone is bound to
> replace the "|" with an "||", a change that is too easy for someone
> to miss.  Once Vatsa is satisfied with the CPU-hotplug aspects of
> this set of patches, if __rcu_pending() still has side-effects, I would
> suggest something like the following:
> 
>         int rcu_pending(int cpu)
>         {
>                 int retval = 0;
> 
>                 if (__rcu_pending(&rcu_ctrlblk, &per_cpu(rcu_data, cpu)))
>                         retval = 1;
>                 if (__rcu_pending(&rcu_bh_ctrlblk, &per_cpu(rcu_bh_data, cpu)))
>                         retval = 1;
>                 return retval;
>         }
> 
> A few more lines, but the intent is much more clear.  And I bet that
> gcc generates reasonable code in either case.
> 
> Or maybe this is just me...

No, me too. For some reasons I can't re-send the patch today, will do
tomorrow.

However, I am not sure anymore that this patch is a good idea. Exactly
because it adds side-effects to rcu_pending().

So, unless somebody on CC: list thinks it may be useful - let's forget
it.

Oleg.
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