linux-kernel - Re: Documentation/credentials.txt

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Message-ID: <20100424001437.GB2589@linux.vnet.ibm.com>
Date:	Fri, 23 Apr 2010 17:14:37 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Serge E. Hallyn" <serue@...ibm.com>
Cc:	David Howells <dhowells@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: Documentation/credentials.txt

On Fri, Apr 23, 2010 at 06:55:33PM -0500, Serge E. Hallyn wrote:
> Hi,
> 
> In the section 'ACCESSING ANOTHER TASK'S CREDENTIALS', the file
> Documentation/credentials.txt says:
> 
> > 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.
> 
> AIUI, that is not actually right any more, is it?  A spinlock does not
> suffice as it does not necessarily imply an RCU read-side critical section
> (anymore).  Of course the spinlock specifically protecting updates would
> suffice, but that's not what this is saying.
> 
> Am I way off base?

You are absolutely correct, good catch!!!

Now, a spinlock still does imply an RCU read-side critical section given
the following configuration options:

o	!CONFIG_PREEMPT

o	CONFIG_PREEMPT && CONFIG_TREE_RCU

o	CONFIG_PREEMPT && CONFIG_TINY_RCU

However, relying on this is usually bad practice, as such code is prone
to failure given the following configuration options:

o	CONFIG_PREEMPT && CONFIG_TREE_PREEMPT_RCU

o	CONFIG_PREEMPT_RT (given the -rt patchset)

And when I get my act together and complete CONFIG_TINY_PREEMPT_RCU,
then CONFIG_PREEMPT && CONFIG_TINY_PREEMPT_RCU will also invalidate
the assumption that holding a spinlock acts as an RCU read-side
critical section.

Did you want to submit a patch for this?

							Thanx, Paul
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