linux-kernel - Re: [PATCH 1/7] introduce atomic_dec_and_lock

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Date:	Wed, 30 Aug 2006 19:25:07 +0200 (CEST)
From:	Roman Zippel <zippel@...ux-m68k.org>
To:	Dipankar Sarma <dipankar@...ibm.com>
cc:	Oleg Nesterov <oleg@...sign.ru>, Kirill Korotaev <dev@...ru>,
	Andrew Morton <akpm@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Pavel Emelianov <xemul@...nvz.org>,
	Andrey Savochkin <saw@...ru>, devel@...nvz.org,
	Rik van Riel <riel@...hat.com>, Andi Kleen <ak@...e.de>,
	Alexey Dobriyan <adobriyan@...l.ru>,
	Matt Helsley <matthltc@...ibm.com>,
	CKRM-Tech <ckrm-tech@...ts.sourceforge.net>
Subject: Re: [PATCH 1/7] introduce atomic_dec_and_lock_irqsave()

Hi,

On Wed, 30 Aug 2006, Dipankar Sarma wrote:

> > > uidhash_lock can be taken from irq context. For example, delayed_put_task_struct()
> > > does __put_task_struct()->free_uid().
> > 
> > AFAICT it's called via rcu, does that mean anything released via rcu has 
> > to be protected against interrupts?
> 
> No. You need protection only if you have are using some 
> data that can also be used by the RCU callback. For example,
> if your RCU callback just calls kfree(), you don't have to 
> do a spin_lock_bh().

In this case kfree() does its own interrupt synchronization. I didn't 
realize before that rcu had this (IMO serious) limitation. I think there 
should be two call_rcu() variants, one that queues the callback in a soft 
irq and a second which queues it in a thread context.

bye, Roman
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