linux-kernel - Re: [patch 8/9] Documentation: Fix invalid rcu assumptions

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Date:	Fri, 11 Dec 2009 21:28:25 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...sign.ru>,
	Al Viro <viro@...iv.linux.org.uk>,
	James Morris <jmorris@...ei.org>,
	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Randy Dunlap <rdunlap@...otime.net>,
	Vegard Nossum <vegardno@....uio.no>
Subject: Re: [patch 8/9] Documentation: Fix invalid rcu assumptions

On Thursday 10 December 2009 00:53:26 Thomas Gleixner wrote:
> Index: linux-2.6-tip/Documentation/credentials.txt
> ===================================================================
> --- linux-2.6-tip.orig/Documentation/credentials.txt
> +++ linux-2.6-tip/Documentation/credentials.txt
> @@ -408,9 +408,6 @@ This should be used inside the RCU read 
>                 ...
>         }
>  
> -A function need not get RCU read lock to use __task_cred() if it is holding a
> -spinlock at the time as this implicitly holds the RCU read lock.
> -
>  Should it be necessary to hold another task's credentials for a long period of
>  time, and possibly to sleep whilst doing so, then the caller should get a
>  reference on them using:

How about changing the documentation to explain why you can't just use a spinlock
or local_irq_disable instead of rcu_read_lock? You explained it well in your
[patch 0/9], but that part had not occurred to me yet and having it in the kernel
sources might prevent more people from getting it wrong in the future.

	Arnd
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