linux-kernel - Re: rcu_do_batch: rcu_data->qlen is not irq safe

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Message-ID: <20060910205827.GD4690@in.ibm.com>
Date:	Mon, 11 Sep 2006 02:28:27 +0530
From:	Dipankar Sarma <dipankar@...ibm.com>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	"Paul E. McKenney" <paulmck@...ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org
Subject: Re: rcu_do_batch: rcu_data->qlen is not irq safe

On Sun, Sep 10, 2006 at 07:08:20PM +0400, Oleg Nesterov wrote:
> rcu_do_batch() decrements rdp->qlen with irqs enabled.
> This is not good, it can also be modified by call_rcu()
> from interrupt.
> 
> So, is it worth fixing? The problem is mostly theoretical.

I think we should fix it even though the problem is theoritical.

> If yes, is it ok to use local_t ? Iirc, the were some
> problems with local_t on some arches. Sometimes it is
> just atomic_t ...

AFAIK, x86 local_t is atomic. Not good.

> 
> Otherwise, we can update ->qlen after the main loop,
> 
> 	local_irq_disable();
> 	rdp->qlen -= count;
> 	local_irq_enable();
> 
> What dou you think?

We should do this.

Thanks
Dipankar
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